Hello, venerable! I — like you — am mostly just inspired to simply practice from these conversations. You make many good points, and I grant you a lot of what you say as valid and relevant to the practice / MN 128. Let me just (semi) briefly clarify my main point.
I think that in the suttas, or meditation in general, the radiance of the mind is used for developing the divine eye. So when the suttas talk about perceiving as though day a radiant mind which is alert and clear and leads to ñānadassana (also referenced in AN 8.64 as referring to the divine eye), I think this is talking about a practice which can culminate in seeing other beings / world systems.
I think that this is the same radiance one naturally experiences when the mind is free of drowsiness and sluggishness and settles into deeper meditation. It’s just that one can play with this radiance and ‘purify’ / ‘project’ it in order to develop certain psychic abilities, especially the dibbacakkhu.
This is the relationship I’m making between samādhi, nimittas, and the dibbacakkhu. As one purifies the mind of hindrances more and more towards samādhi, one may perceive light/forms. These can cause various experiences to arise, from hindrances to more intense meditative experiences involving visions beyond oneself. I think MN 128 is about the general experience of this phenomena, and managing it to stabilize oneself in jhāna.
Keep in mind that one may have already developed jhānas, then started getting more experience with the divine eye, and then they were presented with obstacles arising once they had the divine eye that were blocking their progress. Actually, to give a concrete example (though not one I’d typically quote), in Mae Chee Kaew’s biography I seem to remember one of her main issues in practice was getting lost in the divine eye and not settling into normal samādhi. At one point her teacher threatened to kick her out of the monastery if she didn’t solve this problem; it was a major hindrance, apparently. So this is what I mean by the divine eye before jhāna: one with sufficient samādhi experience starts getting deeper and more profound experiences that become an obstacle.
It’s very possible MN 128 is mostly just about internal nimittas. But I also strongly suspect the divine eye experience is related, as do the commentaries or Bhante Sujato for instance
So personally, I don’t find it unlikely MN 128 is about Anuruddha and friends, who are already experienced practitioners, dealing with trippy meditative visions arising from the radiance of the mind they developed, and the Buddha understanding this. You make a good point about the progress of the Buddha’s awakening, but I don’t see it as a necessary contradiction: the Buddha developed samādhi, then he started developing the divine eye afterward which, as AN 8.64 eludes to, was actually a much more drawn out and involved process of investigation/experimentation. It could have been precisely in this period when the various hindrances or obstacles arose, as with Anuruddha.
Keep in mind that there are several suttas, such as DN 11, which mention that before Brahmā appears, one experiences a bright radiant light. This would imply that seeing other beings occurs in relation to the development of light and nimittas and can naturally happen to an experienced meditator.
Then that mendicant attained a state of immersion such that a path to the gods appeared.
Then he approached the gods of the Four Great Kings … [Divine eye/conversing with deities in AN 8.64]
‘But by the signs that are seen—light arising and radiance appearing—we know that Brahmā will appear. For this is the precursor for the appearance of Brahmā, namely light arising and radiance appearing.’ [More experience of divine eye after light and radiance]
DN 11
He paid attention, applied the mind, and concentrated wholeheartedly on the fate of Magadhan devotees, and sat on the seat spread out, thinking, “I shall know their destiny, where they are reborn in the next life.” And he saw where they had been reborn. …
Then Venerable Ānanda went up to the Buddha, bowed, sat down to one side, and said to him, “Sir, you look so serene; your face seems to shine owing to the clarity of your faculties. Have you been abiding in a peaceful meditation today, sir?” [Reference to ‘peaceful meditation’ as meaning experiencing the divine eye in samādhi]
The Buddha then recounted what had happened since speaking to Ānanda, revealing that he had seen the destiny of the Magadhan devotees. …
Then in the northern quarter a magnificent light arose and radiance appeared, surpassing the glory of the gods. Then Sakka, lord of gods, addressed the gods of the Thirty-Three, “As indicated by the signs—light arising and radiance appearing—Brahmā will appear. For this is the precursor for the appearance of Brahmā, namely light arising and radiance appearing.”
As indicated by the signs, Brahmā will appear.
For this is the sign of Brahmā: a light vast and great.
“We shall find out what has caused that light, and only when we have realized it shall we go to it.” [Reference to understanding the causility/basis for the arising of light in terms of the divine eye, as mentioned in MN 128.]
DN 18
(P.S., DN 18 has some other interesting references such as referring to the radiant vision of the god speaking as ‘nimitta’ — again a connection between this word, light, and the divine eye; it’s a really trippy sutta!)
As for perceiving light not being the ‘flashlight,’ this can be both: the forms (the deities) emitting radiant light (obhāsa), sure. But still I believe that the development of the divine eye is via extending the radiance of the mind outward to see other things. There’s even a correspondence between one giving off light in a subtle body and one experiencing inner meditative light, as MN 127 describes with rebirth.
Before getting too trippy and clunky in describing profound meditative visions, I think I’ll end off here. As far as practice goes and developing deeper samādhi/dealing with hindrances or upakilesas, we agree. And that’s what matters most for Dhamma growth and discussion, more than secondary interpretative issues.
Much mettā and happy practice! 
Strive on!
Vaddha