@1
Nevato is using Omegaman's code for wheel - and it's not possible to show text for missing art without rewriting the code - and no-one is willing to make this I'm afraid.
But ArcadeBliss is working on alternate code and when (if) it will be finished it will allow to do that - and I'll replace the wheel code with the new one.
@2
It's possible - just add your snaps path to your your flyer path definition
CONFIG > EMULATOR > MAME (or other) > FLYER > [edit path, add another path - there can be multiple paths]
if NEVATO (or any theme) can't find artwork in first path, it searches second one, and then the third one, etc...