All destruction of data needs to be moderated. What if the group leader account is hacked? It should not be possible to delete all posts in a group without permission of the website staff.
Interesting. I am not sure I understand what you mean. Do you mean that group nodes will become forum nodes?
Note: If you remove nodes which you associated before, all threads was created belong to that nodes (being removed) will be dissociation. That's mean threads will be public to everyone and no longer display in your group.