No. Just No. Sephiroth is to purely awesome.
When you defeat Sephiroth, Tifa gives you the Fenrir Keyblade.
Yes.
Yes.
If Sora can, anyone can do it.
Unfortunately, I doubt it.
No.
Absolutely he would defeat him in one attack.
Yes.
Defeat Sephiroth.
Sephiroth is an extremly powerful enemy. he is the drak side of cloud basically. In kingdom hearts you can defeat him in the collusem but only cloud can truly defeat him.
They can fight each other in Dissidea, and can defeat each other.
If you play Dissidia, the two can fight. Both can defeat each other.