The answer to the question about them wizards and their tendency to ensure chaos rather than help might lie in Pratchett's essay 'why Gandalf never married'. If I remember it correctly Pratchett back then pointed out how 'normally' it's always the male magic-users that save the day while the women always come second or third place if they aren't the bad guys.
Need to read the article again.
As for the Mary Sue bit. Generally I'm one of the first to point and yell that, though with Tiffany...might be because the book is still for younger readers. But still.... Haven't been able to read it yet myself (try finding the english origina where I live
) but haven't heard that many a good things...