Excellent essay! It is clear and I think the conclusion you arrive at is solid.
I have no problems at all with your interpretation of the verse at DN 11. It is virtually identical to my view of the matter.
The passage at MN 49 is a bit more tricky, but I think putting the verse in the mouth of Baka the Brahmā is the most plausible interpretation. The evidence you have provided is actually quite strong. I don’t think we need look any further for a solution. Infinite consciousness it is.
There is, however, one solution to this conundrum that you have not mentioned. If the passage in MN 49 is instead spoken by the Buddha, then “all”, in my view, would most likely mean all of samsāric existence. Moreover, we have the fact that anidassana viññāṇa means something like “invisible consciousness”. Again, I would suggest that it is reasonable to understand this as a kind of consciousness that is invisible from a samsāric point of view, similar, perhaps, to saying that the mind is “untraceable here and now” (MN 22).
Now the suttas do speak of a kind of samādhi that fits these constraints, namely, the ariya samādhi spoken of in a number of suttas, such as AN 3.32, AN 9.37, AN 10.6-7, AN 11.7-9 + 18-21. This seems to be a kind of samādhi that takes the idea of nibbāna as its object. As such it cannot be accessed by anyone except ariyas.
Do you have any thoughts on this?