I've always felt there was more to Barty Crouch Jr. than we ever found out.
I mean seriously, A) He spends an ENTIRE year pretending to be Moody, and NO ONE figured it out. Dude had to have some serious insider knowledge on Moody no one knew about. B) He spent that entire year actually helping everyone, setting aside the whole actually teaching Harry a lot of stuff that helped him against Voldy, he helped Neville too. Always felt like an interesting quirk.
And as far as why the end of the year. I think it's mostly because Voldy needed that much time to get back enough strength and get everything else figured out. Plus I highly doubt he was just lying around waiting for Harry to get sent to him. I'm sure he was starting to get in touch with his old forces, testing the waters.
That's just me though. I have a much harder time dealing with Barty being Moody for a whole year, than why Voldy decided to not spring on Harry 3 months after he was reborn.