Because the movie was from Obi-wan's point of view and Yoda had not yet taught Obi-wan how to see Force ghosts? We'd have to special up the special special editions so that Luke can't see Obi-wan's ghost until Yoda trains him, but I think this rationalization could work.
Why exactly was Jar-Jar exiled from the Gungans? For being the most stupid and annoying creature in the galaxy?
Well, he was a general in the battle with the droids. I always assumed he was a disgraced officer who had caused the death of some of his men. Then when he was accepted back, they put him back in the army, promoted him to impress the humans, and sent him out to die with glory on the battlefield. (I really can't think of any other logical fate for all these hero generals. Except Madine. He's cool. Shoulda' been a major, though.)
Do they still have podraces on Tattoine during Lukes' time?
Why not?