Love the idea. From a quick look, it would be a good idea to use Low priority querying, with that you can easily bump up the player limit.
char asyncQuery1;//holders for threaded queries
Why? It would have been much easier to use arrays, for example
or even better, an ArrayList.
I would also suggest you to use SteamWorks
. Considering it already needs web server this shouldn't be much of a hassle. You can easily pack all players in one request, and then insert into MySQL from php. Gets rid of any player limits, and much faster. (I'm pretty sure SourceMod has a limit of 50 MySQL queries a second, so anything above 50ish players wouldn't be possible to update a second).