But if an airplane
can roll to 60 degree bank it can be held in the roll and it
will become inverted and then because of the inertia
continue back to upright.

By the time that happens, the nose may well be pointing down. You'd
need to push, and you might not have enough elevator authority to keep
the nose on the horizon. I don't know; I never did it in a 172. I can
certainly imagine other planes that don't have enough authority to keep
it going.

If a Pitts S-2 starts a snap roll and the prop breaks off
the hub, does that mean a Pitts S-2 can't do a snap roll?

If it comes off every time, yes.


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