Sorry for the late reply, maybe you've figured some of this out...
1. The cards just open the way to the Attic, Innocent and Mascot slimes.
2. tbh?? Surprised you haven't killed it yet since that's the ideal strategy for fighting it
3. You just gotta make the most of your buffs here. Have a buffed Julius machine gun as soon as the Grotesqurie's groupies come back, and make sure to keep it bleeding. At the very least, if you can't take out all of its groupies, reduce the amount of health it recovers.
4. hee hee hee consider playing the game with the fewest enemy kills as possible :)
5. Yeeep, you already got everything else, the Devourer's the only thing left.
I haven't played Dragon Quarter, but melessthanthreedid! The Nightmare Slime is based off the nightmares from their game Lucah: Born of a Dream, which they say was partially inspired by Dragon Quarter.