Was Yahweh one of the Moloch Gods?

[I]s Moloch a single God or a group of Gods?

As far as I know (and I admit this is not my field of study), there are very few (if any) sources of Moloch outside the eight mentions of the name in the Hebrew Bible, none of which refers to a "group of gods" or a category of gods; indeed the references are to "the Moloch". Anyways, I would consider the Bible a very weak source as far as other religions are concerned, so I'd rather look at what we know of the Canaanite deities. These are not referred to as "the Molochs". The major deities are El and Asherah. El is just a generic word for "god" (like Ba'al is a word for "Lord"), so that word might refer to an individual deity in the pantheon, or be a title attributed to a specific one, like Hadad, Yahweh, or, indeed, Moloch (which, however, is merely a putative deity in that pantheon due to lack of sources). But even if (and that is a big "if") the frequent mentions of El and the biblical mentions of Moloch should refer to the same deity, that would make the Canaanite deities "a pantheon led by El, who might or might not be identical to Moloch", and not "the Moloch gods". (We don't refer to the Olympians as "the Zeus gods", or the Æsir as "the Odin gods", either.)

I think the author (or, indeed, the translator) is using the term as a convenient (if utterly incorrect) shorthand.

Was Yahweh (Jehovah) one of them, who later rose into prominence?

Whether or not Yahweh originated from that Canaanite pantheon is also a matter of debate still. If he is, he's changed quite a lot from his polytheistic origins, so much so that he would have to be considered a different deity altogether. As for Canaanite El being Yahweh, I personally think that to be unlikely, as what the Canaanites believed about El ("bull god", having two wifes, father of many gods including Hadad, etc.) does not line up with Yahweh all that well.

Q: Was Yahweh one of the Moloch Gods?

In short: NO!

It would be too generous an interpretation to assume the author means 'moloch' to stand in pars pro toto for 'Canaanite gods'. Vivekananda uses the term not as a figure of speech, but literally, as a stand in for 'Israelite gods'. He even calls Israelites 'Jewish', which would be an understandable but ultimately anachronistic terminology. In all, the author Swami Vivekananda seems very influenced by theosophic ideas and other related, integrating inter-religious interpretations, but very late inventions in this direction.

This author here has his very own – and 'special' – interpretations of words and their meaning. Those are quite esoteric, not really compatible with commonly accepted academic scholarship, no matter how diverse these are in light of very scant evidence and contested in most aspects of supposedly 'firm' knowledge regarding Molech.

The passage quoted in question is from volume 1 of Vivekananda's collected works, "Lectures and Discourses, Chicago 1893" in the chapter called "Vedic Religious Ideals". To seemingly complement his mission. In the third volume's "Lectures from Colombo to Almora, San Francisco 1900" chapter "The Mission of Vedanta", he seems to explain this conception of "Molochs" in more detail:

Two such scientific conclusions drawn from comparative religion, I would specially like to draw your attention to: the one bears upon the idea of the universality of religions, and the other on the idea of the oneness of things. We observe in the histories of Babylon and among the Jews an interesting religious phenomenon happening. We find that each of these Babylonian and Jewish peoples was divided into so many tribes, each tribe having a god of its own, and that these little tribal gods had often a generic name. The gods among the Babylonians were all called Baals, and among them Baal Merodach was the chief. In course of time one of these many tribes would conquer and assimilate the other racially allied tribes, and the natural result would be that the god of the conquering tribe would be placed at the head of all the gods of the other tribes. Thus the so-called boasted monotheism of the Semites was created.

Among the Jews the gods went by the name of Molochs. Of these there was one Moloch who belonged to the tribe called Israel, and he was called the Moloch-Yahveh or Moloch-Yava. In time, this tribe of Israel slowly conquered some of the other tribes of the same race, destroyed their Molochs, and declared its own Moloch to be the Supreme Moloch of all the Molochs. And I am sure, most of you know the amount of bloodshed, of tyranny, and of brutal savagery that this religious conquest entailed. Later on, the Babylonians tried to destroy this supremacy of Moloch-Yahveh, but could not succeed in doing so.

And in volume 9, "Lectures and Discourses, Calcutta 1897" chapter "The history of the Arian Race"

Take the case of the Jews. They were divided into so many tribes, and each tribe had a god called either Baal or Moloch which in your Old Testament is translated as "the Lord". There was the Moloch of this state and that state, of this mountain and that mountain, and there was the Moloch of the chest, who used to live in a chest. This latter tribe became strong and conquered the surrounding tribes and became triumphant. So that Moloch was proclaimed the greatest of all Molochs. "Thou art the Java [?] of the Molochs. Thou art the ruler of all the Baals and Molochs." Yet the chest remained. So this idea was obtained from tribal gods.

Further mentions in this exact same strange meaning are in volume 3 "First Public Lecture in the East", volume 4 "India's message to the world", and volume 6 "Mother-Worship".

This view is, shall we say, rather peculiar. Some would surely say this is patently just false, or 'not even wrong'. Neither is Yahweh equated with Molech or counted as 'a moloch', nor are 'all X gods called molochs' anywhere. The phrasing "of all the Baals and Molochs" produces suspiciously few results…

Zero ancient texts, neither epigraphical, archaeological or in copied literary texts seem to allow to draw this kind of conclusions.

Better approaches to the problem of Molech show much more restraint and a far more limited reach.

The evidence for Molech himself is very scant, but all that we do have does not allow for either an equation of: Molech with Yahweh, nor neither a Canaanite or Semitic or (proto-)Israelite pantheon called 'Molochs'.

Molech may have been not even a name of any divinity at all, but a term for a much more worldly ruler. Or a cult practice to a deity of the netherworld, or in reference to the cult of ancestors. Its name related to variations from the root mlk, like Melqart or Milcom, meaning ruler, and the cult involving some child sacrifice.

At best, for the expressed purpose of reading the quoted passage by Vivekananda most sympathetically, the term may have a connection indeed to refer to a small group of rather specialised deities.

Apart from the relevant Wikipedia article on Moloch, the following should ground the reading of the passage quoted in question and establish a more solid foundation of what's known and what's speculated about Molech:

The etymology of the name is uncertain. Most scholars relate it in some way to the (West) Semitic root mlk, "to rule, to be king", either as a Masoretic distortion of melek ("king") using the vowels of boset ("shame"), or as a QaI participle, or as an otherwise-inexplicable 'segolate' noun form (given especially the variations of vowels in the comparative evidence, […]
Contrary to the entire thesis of Molech as a divine name is the proposal of EISSFELDT.

According to his hypothesis, all occurrences of MT molek can be explained as a cognate common noun, so that the stereotypical phrase (as in 2 Kgs 23:10) […] is to be rendered "to cause one's son or one's daughter to pass through the fire as a molk-sacrifice". (Even given this understanding, the etymology remains problematic; the most widely accepted view is that of W . VON SODEN, who suggested a *maqtil-form of the root h/ylk, comparable to mopet and 'ola […]

II. Eissfeldt's proposal has been widely persuasive, as it is founded on a rare combination of comparative literary, inscriptional and archaeological evidence. Both classical and patristic writers testify to a cult of child sacrifice, particularly in times of military emergency, in Phoenicia and at Carthage […]

Finally, it should be noted that an increasingly vocal body of European scholars is challenging the interpretation of the Punic remains as indicating any cult of child sacrifice at all […]

Recent research into comparative evidence has focused on deities named M-l-k (variously vocalized) in places closer to Israel, especially Mesopotamia and Syria-Palestine. A divine name ~Malik is well-attested as a theophoric element at Ebla (third millennium BCE), although little can be determined of his nature or cult there. Amorite personal names from second-millennium Mari include the element Malik, as well as Milku/i, Malki and Muluk (each sometimes with the divine detenninative and sometimes without, so that the common noun, "king", may in some cases be present, rather than a divine name). Of equal or greater interest at Mari are references to beings called maliku as recipients of funerary offerings, although it is not clear whether they are the shades of the dead or chthonic deities. […]

But most significant of all, so far as the study of OT Molech is concerned, is the presence of a deity Mlk at Ugarit. In addition to its inclusion in personal names (vocalized as Malik, Milku and Mulik in syllabic texts), Mlk appears in two divine directories (actually, snake charms), as resident at 'ttrt […], the same location which is elsewhere assigned to the netherworld deity Rpu […] While this collocation does not necessarily imply the identity of the deities, it is suggestive of some close relationship, as is the attestation of beings called mlkm in connection with the royal cult of the dead, along with the better-known Rpu (OT -Rephaim), who appear to be the shades of dead royalty at Ugarit (or of all the dead in the OT; cr. Ps 88:11). Finally, we may note the similar divine names -Melqart of Phoenicia and Milcom of Ammon. While the equation of either deity with Molech is unlikely, […]

In sum, the Semitic comparative evidence yields the portrait of an ancient god of the netherworld, involved in the cult of the dead ancestors (and perhaps their king, given the meaning of the root mlk, at least in West Semitic).

[…]

Among the many questions surrounding Molech and the related cult, none is so perplexing as the god's relationship to other deities (as has been seen already in the examination of the comparative evidence). The Biblical evidence suggests a distinction from Milcom of the Ammonites by specifying that Josiah destroyed distinct holy places for the two (2 Kgs 23: 10-13) and by stressing that Molech' s origins were Canaanite. On the other hand, many have read Jeremiah as indicating an equation with Baal: 'They built the high places of the Baal which are in the valley of the son of Hinnom to cause their sons and their daughters to pass over to Molech, something which I did not command them, nor did it enter my mind … (32:35; cf. 19:5, "they built the high places of the Baal to bum their sons in the fire as offerings to the Baal"). At most. however, this may reflect a popular confusion of the two (or their cults) since elsewhere they are spoken of distinctly […]

[…]

With so much uncertainty, it is no surprise that scholars have combed the OT for additional references and allusions to Molech or his cult, particularly where the MT has melek in a provocative context. With the exception of Isa 57:9 (discussed above), such attempts have commanded little assent. (A recent proposal, involving a passage without an alleged concealed occurrence of Molech, is that of DAY [1989:58–64] regarding Isa 28: 15.18.)

In conclusion. the presence of a deity Molech and of his cult in ancient Israel seems established, although the details of either remain difficult to draw with precision. Based on the comparative evidence, the relatively few explicit Biblical references, and those additional passages which may be defended as relevant, Molech emerges as a netherworld deity to whom children were offered by fire for some divinatory purpose. Less certain, though suggestive, are connections with the cult of the dead ancestors.

— George C Heider: "Molech", in: Karel van der Toorn, Bob Becking & Pieter W. van der Horst (eds): "Dictionary of Deities and Demons in the Bible DDD", Brill: Leiden, Boston, Köln, 1999. p581–585. (archive.org)

Since Vivekananda made the bold assertion that allegedly

Among the Jews the gods went by the name of Molochs.

we might compare this with the actual evidence we have to quite contrasting ends. It seems that nowhere in the surroundings of the area or cultures in question was 'Molochs' used ever to refer to 'all/the gods' or a whole pantheon. Among the Israelites and in ancient Hebrew we see, for a very brief summary, different words used:

GOD

I. The usual word for 'god' in the Hebrew Bible is elohim, a plural formation of 'eloah, the latter being an expanded form of the common Semitic noun 'il (-Eloah). The term elohim occurs some 2570 times in the Hebrew Bible. with a variety of meanings. In such exprcssions as "all the gods of Egypt" (Exod 12:12) it refers to a plurality of deities—without there being a clear distinction between these gods and their images.
Far more frequent is the use of the plural with reference to a single being: —Chemosh is the 'elohim of Moab (I Kgs II :33); the plural here is a plural of excellence or of majesty […]
Though having the generic sense of 'god', the term is also used in an absolute sense ('the god'. e.g. Gen 5:22) whence it developed the function of a proper name ('God'): when an Israelite suppliant says his soul thirsts for 'elohim he is not referring to just any god but to Yahweh the god of Israel (Ps 42:3). Since the Israelite concept of divinity included all praeternatural beings, also lower deities (in modern usage referred to as 'spirits', 'angels', 'demons', 'semi-gods', and the like) may be called 'elohim. Thus the —teraphim (Gen 31: 30.32), anonymous heavenly beings (Ps 8:6; LXX αγγελοι), and the 'spirits of the dead' (I Sam 28:13) are referred to as 'gods'. […]

Other Hebrew words for 'god' are 'el […] and 'eloah. Though both are used as proper names ("EI your father", Gen 49:25; "Can mortal man be righteous before Eloah?", Job 4:17), they can also have generic meaning: in the latter case they are more or less interchangeable with 'elohim.

— Karel Van Der Toorn: "God (I)", in: Karel van der Toorn, Bob Becking & Pieter W. van der Horst (eds): "Dictionary of Deities and Demons in the Bible DDD", Brill: Leiden, Boston, Köln, 1999. (p352–365. PDF)

Direct detail to subquestions:

Q: … that Moloch is actually a single name of a Canaanite God. So is Moloch a single God or a group of Gods?

That is far from certain. Candidates for 'what does 'Molech' mean' range from name of a deity, to a small group of deities, a 'ruler' or even just 'a certain practice' of worship.

Q: Was Yahweh (Jehovah) one of them, who later rose into prominence?

No. What seems rather certain here is that this is not the case. Yahweh may be historically interpreted as an initially rather minor god of the area that indeed acquired all other divine aspects of all other gods, necessarily including all aspects of whatever aspect of divinity Molech represented.
But we have zero evidence for the Hebrews/Israelites referring to 'all their gods' as "Molochs". To the contrary, all direct evidence for Molech are from 'the bible', and in there we see that exclusively (aspects of) 'some other('s) gods' are meant with this term. References to any "Moloch-Yava" and the like are absent from the texts we have.

Note that if we simply replace Vivekananda's word "Moloch" with 'god' in most of his texts (and ignore his assertions about 'Jews called this that'), then his descriptions makes a lot more sense…
There seems to be at least 'an error' in his analysis. Whether or where this 'special' word usage is ranging between unintentional conflation or even intentional polemic is up the reader to decide.

Addendum: While there probably will be no way to simply read Vivekananda's word usage as anywhere 'correct' for our present understanding of ancient texts, word usage and cults, understanding Vivekananda's heavily intertextual reasoning on this might be advanced quite a bit by re-reading —among others, more 'modern' influences— Milton's (Paradise Lost) and Tennyson's (Howl II, The Dawn) treatment of the subject of Molech, read together with biblical hints on child sacrifice (like beginning with 'binding of Isaac') and from surrounding cultures, perhaps one might begin to understand why his audience seems to have understood what he meant more easily. (Cf. — Heath D. Dewrell: "Child Sacrifice in Ancient Israel", Explorations in Ancient Near Eastern Civilizations 5, Eisenbrauns: Winona Lake, 2017. As well as — Francesca Stavrakopoulou: "King Manasseh and Child Sacrifice Biblical Distortions of Historical Realities", Beihefte zur Zeitschrift für die alttestamentliche Wissenschaft 338, deGruyter: Berlin, New York, 2004.)

Another quite recent analysis traces Vivekanada's views back to another source:

[…] the article turns to Vivekananda’s views of Christianity, for which he had little affection, and the Bible, which he knew extraordinarily well. […]

Vivekananda repeatedly asserted the puzzling claim that the tribal god who became Yahweh was originally one of the Canaanite deities, Moloch (sometimes translated Molech), reputed in Hebrew scripture to demand the sacrifice of human children.

On this basis, he claimed, “Human sacrifice was also a Jewish idea and one that clung to them despite many chastis*m*nts from Jehovah.” Animal sacrifice was a fixation of the Jew, he insisted. He made the indefensible assertion that the practice was inimical to Indian belief, conveniently forgetting the prevalence of animal sacrifice at locations like the Kali Temple in his native Kolkata. Vivekananda’s view seems to derive from the German antisemitic author George Friedrich Daumer, a close friend of atheist philosopher Ludwig Feuerbach, who espoused a belief in Jewish ritual cannibalism and equated Moloch and Jehovah (Yahweh) in his 1842 The Fire and Moloch Cult of the Ancient Hebrews.

— David J. Neumann: "Christ as Yogi: The Jesus of Vivekananda and Modern Hinduism", Church History, Vol 90, No 1, 2021, pp.117–140. doi

For comparison: the modern Jewish Virtual Library: The Cult of Moloch, Wikipedia: מולך, versus Daumer's "Der Feuer-und Molochdienst der alten Hebräer, als urväterlicher, legaler, orthodoxer Cultus der Nation, historischkritisch nachgewiesen durch G. Fr. Daumer", 1842.