1. The oil will burn off, if you are hell bent on getting it off, try brake clean and steel wool (when the exhaust is completely cold!)
2 - 3. I recommend to everyone to hit up quadratec and buy the rear extension with the bushing already pressed in and machined...it is an extra $15 well spent, once you have the extension in hand, tap your new seal in with a rubber mallet, I always lube the seal with ATF prior to installation.
Be sure to mark the orientation of the rear extension, it only goes on one way (the one ear that is off center relative to the casting ribs is in the 10 o'clock position.