Kerry wouldn't be the nominee without 9/11 IMHO. It would either be Gephardt (unless he becomes Speaker of the House in 2002 and focuses on that), Dean, or Edwards.
I think it would have been Gephardt or Edwards as you said, and I also agree without 9/11 Kerry would not be the nominee, on the other hand I don't think it would have been Dean either.
At anyrate assuming Kerry was still the nominee in 2004, I'm tempted to think Bush would have done better than in IRL because he wouldn't have had the peace crowd opposing him on the one hand and on the other hand I think Kerry was too liberal to flip the states he didn't win in IRL.
On the other hand in this scenario I could see Gephardt or Edwards beating Bush, which I'm not convinced of had they gotten the nomination in IRL.