You are allowed  to use the surnames Potter, Malfoy and whatsoever. As long as it's not in combination with the original name (so Harry Potter fe) and as long as you don't describe yourself as related to any of the characters from the books. Potter for example is just a regular surname and so are a lot of the others.

I believe it only works of you still have every single item from the different shops in your invetory, and not if you've given it away or deleted it for example.

Normal users are sometimes allowed to use those tags while given permission from the Minister, Headmaster or so on. Making them only available for people with the 'special' ranks, will unable that feature.

If you look on EU for example, there are tons of graduated (ex-staff members) that are most likely willing to play in a Quidditch team.

People paid for their clubs so it wouldn't be fair if they were just removed like that just because their club doesn't meet the standard of x amount of members. I agree it's shitty there are useless clubs, but this to me is not a solution.

I agree with this, I think management should be able to start clubs for their Quidditch team for example to get better communication. However I still think it's good that users will have to pay for a club with real money, because then users will rethink their choice to make a club twice and we won't have a list with 100+ clubs. This way it'll keep things within limits.

This idea has already been proposed: http://feedback.worldofpotter.eu/topics/385-leave-a-club/

This idea has already been proposed: http://feedback.worldofpotter.eu/topics/385-leave-a-club/