1. VBCC definitely sounds like the best idea.
2/3/4; So, actually what i'm wondering here. do you plan a RTG system Ala CGX/P96 that patches the hell out of preexisting Graphics/Layers/Intuition. Or a AROS-derived-where-possible clean replacement with 'RTG functionality' integrated?
The second option sounds like the sanest choice as that also opens up the possibilities of further development of those components.
In any case, if you have the time to do this (alas i don't) you are a hero :-)