Your 1) is not related to reverse engineering at all. Just low-level programming.
What EXACTLY needs to be reverse engineered to "make old school games" or "fully functional operating system"?
It's an ARM and the GPU has some API.
What EXACTLY needs to be reverse engineered to "make old school games" or "fully functional operating system"?
It's an ARM and the GPU has some API.
EDIT: Ok, I think I see what you mean now. I believe your question is about what is the appeal in doing this stuff. It's pretty much the same reason why someone would build a chair or a table from scratch versus buying one at IKEA and building it. The former gets the enjoyment from all of the journey from a piece of wood to the finished chair and from building exactly the chair that they want and the latter is probably more focused on the chair itself. END_OF_EDIT
As for the rest, I believe that some answers are already in the post and in the comments that I and other people made.
I may have misunderstood what you meant and if that is the case let's restart from scratch:
Do you want to program a raspberry pi at the hardware register level, without any APIs ?