Having my own FC just two days I'll try to answer some questions by what I think it is:

1. Don't know yet about crest ranks itself but Leader / Full is you with full access.
2. Not sure about that either but I think as you level up you'll gain credits which you can use on those actions.
3. Well - by leveling each of it's memebers - you see the progress bar on top right with star next to it - as your members level up it will fill up as well.
4. You can create your own membership / permission levels with custom set authorizations so you can assign specific FC members with dedicated access to individual features if the deafult 3 groups are not sufficient for your needs.

I plan to explore a bit more about FC tonight so if I'll find something to add I'll update the post here.