For the first topic, you’re right, it seems like you can’t link pages to a collection. Which is strange, because I remember being able to do it some weeks ago. Maybe it’s a bug with the new update, or maybe I’m wrong and it was never possible.
So yeah, I’m sorry to tell you I don’t know a way to achieve it.
For the second topic, the fastest way for me to unlink an object is to link the object to another object, and then going to the original parent and deleting the access. To better explain myself, I will show you on a video:
I don’t know if there’s a faster (and more convenient) way to do it, but for now is the solution I can give you