Maul needs to die (again) in the Clone Wars series. That would make the most sense thematically.
Absolutely. Whatever they have planned for them, they shouldn't assume that the core film audience is out watching all of the Clone Wars episodes.
For consistency Obi Wan must kill Maul "again." Otherwise there is so much material set in stone out there that would suddenly become incorrect (i.e. Q: Star Wars trivia game: Who killed Darth Maul?)
However I hope they take their time correcting this. After the four partner this year I'm convinced bringing Maul back from the dead was a stroke of genius. Perhaps we'll even learn how he was able to keep alive. Being cut in half is not something most bipeds should be able to... ahem... 'walk away from.'