>then it's pretty hard to get back in.

It's really not. Even if they aren't recruiting new members you can buy an account or invites. You could also prepare alternate accounts ahead of time.

Not all of them do, but yes. Since it is the client who send all the info (bytes sent/received, version, etc), anything can be spoofed. The tracker, having access to the stats of all the client on a specific torrent can find "unusual" client (for example, a client that report a lot of bytes sent while no other client report downloading/receiving anything, or the opposite). But due do quirks of the networks, delay and possible lost stats messages, they still have to be pretty permissive of small discrepancies. Also, all use an ID in the tracker url for each client to discriminate them and associate them with their account on the website. And some are not very careful about the ID generation algorithm, leading to being easily able to find other account peer ID, and so you can use them to download to preserve your ratio. I even found at least one private tracker that did not care if you passed a non-existent peer ID.