I see. The appearing rates are pretty similiar for all types. There are no enemies that are much more uncommon than others, excluding rare encounters of course. And yes, fairies for exsample are mainly in forest areas but may also appear in a few others.
No they should not disapear, only be pushed backwards. They disappeard in earlier versions but that often resulted in games crashes so I rework it.
I can't quite remember, I think you needed to get the app store working again, and in there it can be bought. Which gets restored as a reward for a quest if I remember correctly.