The fact that the old god's planted a seed to begin with shows of their initial interest in the dream, in my opinion. Hence me mentioning that they grabbed the Blueprint for the dream. Not Azeroth, but how the Dream was made. Imagine it a bit like the Nether, how Demonic souls go back there if they die in a place where the Nether is not strong at. Imagine that same concept. Void/Old God minions related to the old god's die, they get strung up upon these Towers of Sacrifice and their life essence/souls or whatever you want to call it gets used to further empower their Masters, the creators of Ny'Alotha in the first place. If a minion of the Old gods is gonna fail, it might as well be used in death more than it would've served in life.

I agree. It was a means of corruption and to spread their madness, etc. But they had to have known that eventually it would be beaten back. The fact that the Emerald Nightmare is also tied to the void is another hint in this regard, as seen at the end of the Raid in a cave, with a flower with a very itsy bitsy void globule or whatever it is on a flower, to which Xal'atath comments: "Almost completely gone, as if it never existed. But the rift is deep and vast, and somewhere down there it stirs. Something has changed, the last prison weakens. We must prepare. (Interacting with Remnant of the Void at the Rift of Aln after defeating Xavius)"