The Nullstone wouldn't serve much purpose since it doesn't just kill voidsent, it specifically sends them from the Source back to the Void. It would only help us if Golbez actually does manage to cross worlds.

On the topic of mandatory sidequests I don't think the Mhach raids really need to be relevant here, but it is extremely odd that we're doing this whole Void storyline without Cylva and Unukalhai. Which is why I think the restoring the Thirteenth stuff Y'shtola was talking about will be a sidequest itself, and require completing their side quests too. Both of them actually have new dialogue after you finish 6.2 saying to come back to them if you get any more information.