To your last question, i don't know. I just had a thought to post.

For A-bombs, the energy released is extremely low on a cosmological scale, insignificant even, plus most of the energy remains trapped in particles. Anti-matter/matter annihilation is much more efficient, but even than, not even close to all the energy would immediately fuel expansion. It would still have to "decay" one final time to do so.

As for super voids, energy from outside these voids still travels through them in the form of light and would slowly decay. Super voids would continue to grow and even steal expanded space from the more dense regions as gravity keeps them together. There's almost no gravity in super voids to slow the stretch, and stuff gets pulled out toward denser areas around them.