The answer to #2 is related to #1. Once you reset your database during the academic year (i.e. July 1st to June 30th), your performance statistics are locked (i.e. they will no longer update). The rationale is that people who are seeing questions for the "second time" (i.e. after resetting their database) will artificially perform better than users who are seeing the questions for the first time during that academic year. Thus, we are trying to make the performance statistics more meaningful.
You can imagine a user who decides to reset his database every week prior to the final exam. Obviously, his performance is going to be better on the questions, which will skew the entire performance database and make them less meaningful for everyone else.
So, if you do not want your performance statistics to be locked during the academic year, then don't reset your database.
The performance statistics are then reset July 1st of the new academic year. In other words, resetting your questions database during your first log-in after July 1st will not lock your performance statistics.