Deleting Inactive members
I have considered making it members only to download, as I think that it is nice to be able to use some of the site features which you don't get as a guest.
I also think a member is more likely to leave a comment (as we have no guest posting) so really the barrier is in the wrong place. There is no incentive to post and thank a creator if you can just dl as a guest, and to sign up to say thank you is less likely than to sign up and get the dl and say thanks etc while downloading as it is so easy then.
I'm all for deleting inactive members if it's causing any bother for you. If it isn't... then entirely up to you.
Also, if I recognise a users name from MTS or something I won't delete them either.
I think what you're doing sounds like a good plan