I believe the entire thing fuses, all layers. I know that at least the cambium layers definitely need to fuse.

Exactly....that's my point, and why I asked. Just how much fusing takes place, and where? I think, if I were to bet, that it is only at the cambium layer where fusion takes place. And if this is true, what prevents the un-fused, tissues from rotting internally, from lack of nutrition? There has to be more to this story.

Growers of grafted plants: Do you ever experience any sudden graft-failures?

Frank

All I know is graft plant won't grow as fast as original plant. Graft will take longer to collects energy to care for itself.
Yes, the graft is easy to die and needs more care. Easy to fail in winter than original no graft plant.
Marie