Hmm

1. Maybe, maybe not. Can't say for sure.

2. If the use of levity (better term than antigravity, IMO) instead of magnetism does make fusion more efficient, then I would have to say yes.

3. Fusion requires very high pressures, so I don't really see how an EMP could cause a runaway reaction without also disabling the containment and making fusion impossible. If the EMP had any part in the destruction of the Black Star, it would probably have been by disabling the containment on some antimatter, or screwing up something else volatile.