Dhatu-vibhanga sutta question?

There is no "external" consciousness (SN 12.19) in relation to individual experience.

At Savatthī. “Bhikkhus, for the fool, hindered by ignorance and fettered by craving, this group (kaya; comprised of five aggregates including consciousness) has thereby originated. So there is this [internal] group and external mentality-materiality (nama-rupa): thus this dyad. Dependent on the dyad there is contact. There are just six sense bases, contacted through which—or through a certain one among them—the fool experiences happiness and suffering.

SN 12.19

There is consciousness of external objects, such as consciousness of external earth, wind, fire, water & space element and consciousness of external mentality & physicality (nama-rupa; per SN 12.19) but there is no consciousness of external consciousness.

For example, when experiencing foolishness externally, the wise man experiences the foolish mentality of others, externally. But the wise man cannot experience the consciousness of another. For example, the wise man cannot experience what another person is seeing, hearing, smelling, etc. They can only experience the mental feelings & sankharas of another (such as experiencing externally another's confusion, ignorance, happiness, sadness, lust, anger, etc), which is why the suttas refer to external "citta" rather than external "vinnana":

They understand the minds of other beings and individuals, having comprehended them with their own mind. …

Parasattānaṃ parapuggalānaṃ cetasā ceto paricca pajānāti. Sarāgaṃ vā cittaṃ ‘sarāgaṃ cittan’ti pajānāti, vītarāgaṃ vā cittaṃ … pe … sadosaṃ vā cittaṃ … vītadosaṃ vā cittaṃ … samohaṃ vā cittaṃ … vītamohaṃ vā cittaṃ … saṅkhittaṃ vā cittaṃ … vikkhittaṃ vā cittaṃ … mahaggataṃ vā cittaṃ … amahaggataṃ vā cittaṃ … sauttaraṃ vā cittaṃ … anuttaraṃ vā cittaṃ … samāhitaṃ vā cittaṃ … asamāhitaṃ vā cittaṃ … vimuttaṃ vā cittaṃ … avimuttaṃ vā cittaṃ ‘avimuttaṃ cittan’ti pajānāti.

MN 119

Note: when the suttas refer to "external consciousness", such as in SN 22.59, this external consciousness is only imputed (such as if saying: "my friend had a heart attack and lost consciousness"). It is not something directly experienced.

Any kind of consciousness whatsoever, whether past, future, or present, internal or external, gross or subtle, inferior or superior, far or near, all consciousness should be seen as it really is with correct wisdom thus: ‘This is not mine, this I am not, this is not my self.’

SN 22.59

Perhaps it's because the first four elements at least, probably the first five, are included in rupa -- but consciousness is separate from rupa.

SN 12.67 says that "consciousness" and "name and form" are like two bundles of reeds leaning on each other (i.e. they are not part of the same bundle).

Suppose there were two bundles of reeds leaning up against each other.

In the same way, name and form are conditions for consciousness. Consciousness is a condition for name and form. Name and form are conditions for the six sense fields.

The space property may be either internal or external...

There remains only consciousness: pure & bright...

Possibly because there're 2 ways of classification: space and time. The Rupa-related components(earth, liquid, fire,...) were space-wise, while the Non-Rupa-related components(consciousness, equanimity,...) are time-wise. Notice the Buddha didn't reject the space-classification of consciousness since He did mention internal/external consciousness in other suttas. He just didn't use it here and the phrase "There remains only consciousness" indicates this time-wise classification (ie. there might be other consciousnesses, external, not pure, not bright, etc.. until there remains only the pure&bright consciousness).

The five elements (earth, water, fire, wind and space) are physical. They are described as internal, when they are part of the body, and they are described as external when they are not part of the body.

Consciousness, the sixth element, here, is purely mental. It is of the mind - that's why it's only internal, not external.

Pure and bright definitely refers to this verse: "Luminous, monks, is the mind. And it is defiled by incoming defilements." (AN 1.49), as Andrei has already mentioned.

The two truths... relative truth and absolute truth.

Look into that for deeper penetration into your subject matter. For example; relative in the moment could say my my isn't the sky a bright blue today? Relating to all of those words? Why yes it is!

Absolute truth no it isn't as there is no sky no blue etc etc.

Relative truth is attachment, grasping.

However both are and are not correct as truth at the exact same time... creating a paradox. Nagarjuna is a good commentator on the two truths.

For the sake of peace one truth is samsara and one truth is nirvana; The yin and yang or inseparable whole impermanent and always in motion as the seventh jhana/consciousness. Cease to grasp and the infinite rounds of birth and death due to the consciousness skandha falls away into nirodha samapadi or the 8th jhana.

That's the very meaning of: "Fall down seven get up eight." as a koan.

If you look at original Pali, what they translated as "consciousness" is actually "vijnana" - (discriminative) experience.

The point of the sutta is, as you meditatively investigate what seems to be your "I", you progressively exclude all physical elements as obviously not "I", until you are left with just experience itself. Then you investigate that experience and realize that it depends on the sensory input, which depends on attention.

When you don't pay attention to any sensory input, experience stops. What's left is called "equanimity" ("upekkha"), characterized as "luminous" or "radiant" ("pabhassara"). See Pabhassara Sutta for the canonical reference to "luminous mind".

Then Buddha explains that although this equanimity can be shaped into any other meditative state, including the formless jhanas, the meditator understands that any such state would be contrived and conditioned and therefore finite and prone to suffering.

This realization leads to letting go of desire to attain any state different from what is presently unfolding. The resulting state is pure suchness providing no basis for suffering to arise. The meditator realizes that this indeed is the culmination of the path, "the knowledge of the ending of suffering".

So, the "experience" is not analyzed as internal and external because we are talking about meditator's own experience as he tries to find who he is by separating from everything that is conditioned and unreliable. Experience (vijnana) itself is conditioned on sensory input and is therefore unreliable, not suitable to be called "I" or to be our basis for eternal happiness. Which leaves us with nothing to rely on, nothing to stand on - and this final realization being the only unshakable truth, acceptance of which gives unconditional freedom from any mismatch of expectations, - is the very Nirvana the Buddha is leading us to.