That just means you need a second nuke to blow a hole that allows the gas to escape & lave to flow in, and a 3rd nuke to collapse the hole before the lava gets too far. It's all there in my book, "How to nuke your way out of any problem"

Big "if", but hypothetically, at least gas flows in a way that the pressure could be relieved.

The whole reason supervolcanoes are what they are is that the material can't sufficiently flow faster than pressure builds.

Converting that material to an escaping gas at scale would be an unbelievably successful technological achievement.

You could just create another bigger void and let the stuff vent into that

Considering they are mostly anorganic substance, I'd guess you can make them boil - you'd get lava.