1. To fully unlock a Fairy's gallery, you need to collect 4 essence. Each essence collected will unlock about 1/4 of that Fairy's gallery.
2. Sticking to Oral only with a few exceptions (Slime girl > liquid form)
3. Yes. Next update will be adding 2.
4. Not during gameplay. You can go into the gallery and find pictures of Amelia with a big belly and Consensual Predators as only prey if you want to construct an image this way.