1. It's on the tentacle path, after the Goat dudes village.
2. Which one? Being defeated by small slugs or the giant slug will trigger the event. For the giant slug, there is only 1 event. For the small slugs, if Demyra is perverse enough, she will be submissive instead of rebelling.
3. This one is a bit complicated... Demyra has to be dragged to the tentacle altar at least once, for that you have to be defeated by the ambushing tentacles at least once... when she escapes, talk to the stone guy and he will give her a quest to offer herself as breeding stock to the tentacles. BUT, in order for Demyra to actually get that option, she must be perverse enough, if memory serves me right, she needs high enough vaginal intercourse experience, I m not sure what the value is. Once you get that, you can either teleport to the altar or be dragged there, use the interact button and Demyra will get the option to offer herself. In exchange, the ambushing tentacles will no longer attack her.
I know it ain't very intuitive... also, I think it might not work if you are too advanced in the game... mind that this game is in development, so quirks happen...