[[WMG: Igor is two people.]]
That's why his hump changes! That's also how he got down from the tower quickly in the reanimation scene and why his name is pronounced two different ways. It's totally logical.
[[WMG: The Abnormal Brain is Victor's.]]
Because the townsfolk thought him completely insane, of course his brain would have been both kept for further study among the (apparently very large) neuroscience community in Transylvania, and it also would have been labelled as "Abnormal". Of course, because Victor's insanity was much more subtle than that displayed by the monster, it must have been an error on the part of Frederick while inserting it into the body that caused the imbalance. When the Operation took place at the end it merely restored Victor's brain to its former state of slightly mad genius. The main thing this explains is how the new monster remembers the tune played throughout the film - ''he remembers it from when he played it to his monster''.
* Actually, that could be a sensible notion since he DID also survive a fire, possibly explaining his fear of fire as the Monster.