Why are these called "The All"?

Buddhism's methodology is trying to make people aware of what they are not instead of giving them informations of what they are. But this doesn't mean that Buddhism denies some of the other traditions view of the universe. I watched Thich Nhat Hanh's dhamma talks and he said many times that "we are waves in an ocean". So in a practical sense, every meditator can come to this point. Especially If you reach to the really advance stages in your spiritual path, you start to be more aware of the spiritual essence of things than their form identities all the time. And ofcourse, the experience of Nibbana itself will lead people to this "oneness" idea easily. I think there is nothing wrong with that If you are deeply involved in the practical side of the Buddhism: Which is meditation/mindfulness.

I think the reason for Buddha's methodology of telling people what they are not is to avoid people's tendencies to make the spiritual path to a belief/worshipping based religion. For example he used words like "Emptiness". "Nibbana". You can not worship Emptiness, Nibbana..You can only worship the god. Maybe Buddha's methodology saved Buddhism from transforming to a dry/harmful religion like some of the other religions that most of us know. This doesn't stopped many Buddhists misunderstandings of the core of the Buddhism but the core of the teachings of the Buddha is still alive and being practised by many monk and lay people in the world. And this leads to some people's complete freedom from suffering. So these things proves that this methodology works well. The vast majority of the humans will never get involved with true spirituality but that is not Buddhism's fault.

1. Why are these called "The All": the eye & forms, ear & sounds, nose & aromas, tongue & flavors, body & tactile sensations, intellect & ideas?

Ven. Bodhi's note citing the Commentary to SN 35.23:

Spk: The all (sabba) is fourfold: (i) the all-inclusive all (sabbasabba ), i.e., everything knowable, all of which comes into range of the Buddha's knowledge of omniscience; (ii) the all of the sense bases (aatanasabba), i.e., the phenomena of the four planes; (iii) the all of personal identity (sakkyasabba ), i.e., the phenomena of the three planes; and (iv) the partial all (padesasabba), i.e., the five physical sense objects. Each of these, from (i) to (iv), has a successively narrower range than its predecessor. In this sutta the all of the sense bases is intended. The four planes are the three mundane planes (sensuous/form/formless planes) and the supramundane plane (the four paths, their fruits, and Nibbana)."

The Comy.'s explanation above could also help answering your 2 subsequent questions.

0. History base knowledge about wrong and right view: See SarathW's Answer.

Then:

1. Why are these called "The All": the eye & forms, ear & sounds, nose & aromas, tongue & flavors, body & tactile sensations, intellect & ideas?
2. Why someone trying to explain "The All" as being something else would fail?
3. What does "it lies beyond range" mean?

The wrong view has the wrong condition(s). The right view has the right condition(s). The all right condition is the dependent origination, because the conditioner and the conditioned relate each other in balance, nothing left out of the cycle of the condition.

the dependent origination explains "Thus arises this whole mass of dukkha".

Each conditioner of the dependent origination causes the whole conditioned effect. For the example, ... the (Six-)Saḷāyatanas-conditioner causes the whole conditioned Six-Phassa; The Six-Phassa-conditioner cause the conditioned six-vedanā ...

So, if one, who questioned, explains the All, whole, into the dependent origination as the Buddha did, that one is able to explain the whole from the beginning to the end follow to the dependent origination such as the Buddha can explain every view in Sutta. Tī. Sī Brahmajālasuttaṃ.

But if one, who questioned, explains the All, whole, out of the dependent origination, that one would be unable to explain, and furthermore, would feel hard to answer (fail). Why? Because it is beyond that one's abilities, he exactly answers the wrong condition (fail), i.e. God condition the being.

SN KhandhaVagga and SN ṢaḷāyatanaVagga, such as SabbaSutta, come after SN Nidānavagga, the dependent origination because they are the whole elements which cycling in the dependent origination.

Buddha's time in India there were many views about the world. This is similar to saying nowadays "The all mighty God" etc. These views are discussed in the Brahmajala Sutta. These wrong views lead you to a wrong destination. However, according to Buddhism the right view lead to Nibbana. What Buddha saying is nobody can comprehend anything beyond six senses.

https://www.accesstoinsight.org/tipitaka/dn/dn.01.0.bodh.html

Why are these called "The All": the eye & forms, ear & sounds, nose & aromas, tongue & flavors, body & tactile sensations, intellect & ideas?

What else is there? Note: Here the word 'ideas' is a loose translation. The pali word is 'Dhamma'.

Why someone trying to explain "The All" as being something else would fail?

Because anything you can see, hear, smell, taste, touch and think comes under these categories.

What does "it lies beyond range" mean?

It means if you claim that there is something else other than these, it will be limited empty words and not backed by direct knowledge.