Technically that would be Lenga, but you can get Sol after Lenga too so it's one or the other basically.
Not currently but we'll add a few in the future.
There are the random events which can randomly happen in the castle (you can also trigger them using the scrying pool in Cervius's bedroom but for the scenes you're missing, you can look at the condition(s) in the gallery)
Yep, CGs and npc faces are handled by Coconut while I draw the portraits and animations ;)
You can unlock an additional questline through the brothel