Hi ptoing,
unless I'm overlooking something, here's a little suggestion to save you lots of headaches.
Since you committed to update the reference picture whenever required, while at it, you could just paint the newly claimable tiles in green so that the claiming rules could be restricted to these:
1. Only green tiles can be claimed.
2. Requests are satisfied on a first-come-first-served basis.
3. You can not claim a tile which is adjacent to one you made earlier.
4. After claiming a tile it has to be done within 72 hours of the claim post. Failure to do so leads to freeing of said tile. Complaining won't get it back. Tough cookie.
The only disadvantages are that:
* we aren't told expressly the policy applied to select the claimable tiles;
* theorethically, we can't make predictions about claimable tiles based on the "DONE" messages by others.
Sorry for adding meta-discussion to the thread, but I thought that this could make everybody's (and especially yours) life easier.
saimo