once you kill or make the choice with your denizen, do imps and such stop spawning? If they're created by the denizen. And if so, does that mean that after dealing with them, there's no way (or very few ways) to get more grist? I guess after getting the horde it's not that important.

Arguably they SHOULD stop spawning? You need the Grist Horde for the Ultimate Alchemy, so I wonder if you can access it at all after the Genesis Frog has been created. Hmmm. Quite unclear!