No, you cannot. You can apply to be a member, but not apply to be a leader.

This is something that you have to ask your plot leader about, as this is a thing for individual sites.
Feedback is more a thing for every site.

In what way does this go more in depth?
Topics are used for interaction between people and not just yourself, and you are not really allowed to use a lot of NPC in topics anyway, but you can do that in the backstory.

This can be done i either hallways or classrooms

Exactly! The Past forum is to interact with other people, if you perhaps wanted to have a chance to look at their childhood with another person :D
I have a few relations with people, which would benefit greatly from having topics in the Past :D

Isn't it only the DK site that uses it for food?
I don't really know of any other site that uses it. The @all tag is used to convey important information to the users, if for example something important is happening or notifications don't work.

librarians write books for the library, so perhaps apply for that team

the dormitories are already there though

I think it is a good idea, but I don't think it should be on profiles and only be something, which teacher's should be able to see

up :D