1) Because music boxes require some spriting (which is usually the hardest part about any new content for the remnants dev) and some surprisingly not that simple coding for little to no payoff.

2) It is currently still to be decided, the latest plans are to have it in the middle of an another planned brand new biome called toxic wastes, which will take up the entirety of jungle biome's lava layer.

3) Infernal Stronghold is a WIP structure that is going to replace all of the underworld's structures as well as providing some additional content.

Both Vault and the Stronghold already generate with the experimental worldgen config option on, though, obviously, they are extremely barebones and unpolished. Check them out for yourself.