Thank God Kuja the user ain't here. He might be inclined to kill you.

1. Sephiroth's Jenova Cells provoked him. How else can you explain him suddenly going insane? It's a crappy plot, but Jenova's influence on him is evident.

2. Trance is normally a Limit. Like Kuja said, a normal Trance wouldn't work. So, he used the Invincible and the souls it had to create a sorta Super Trance. As far as we can tell, he can maintain this Trance for an extended period of time. It might even have been permanent, but his near-death state removed it. A normal Trance is nothing compared to his.

3. Sephiroth has technically two forms. Human can count, but Safer and Bizarro Sephiroth forms aren't really taken into consideration. We're just assuming he has all the powers he had while in those forms, but he can stay in human form. I doubt he would want to be in Bizarro or Safer Sephiroth form since I doubt either form has alot of mobility.

With Curaga and Reflect, Kuja could deal with Seph's magic. His physical attacks are the ever present problem. Like I said, he simply needs to use magic to keep Sephiroth at bay. Flare Star, Demi, Flare, Holy, and of course, his sorta Super Ultima. His Ultima is unique and the Ultima variations in other FF's cannot compare. I say he could win, but I await your reply.