Since no one has answered this and I'm guessing the only person who can is Bail, I'll try and answer with educated guesses and what I know.
1) My guess is that if you run a threaded query and the connection dies before it resolves, then it'll lose all data but still call the handler function.
2) I don't think the queries queue up at all. As said in the wiki, the queue is "push one, resolve one, pop one."
3) I don't think they queue here either, I think the plugin simply halts there until it recieves a response.
4) The API is better abstracted to indicate what SQL is really like. It also has some more advanced features, such as being able to run 2 modules at once (MySQL and SQLite) and threaded querying.