1-Shadow either killed himself or started a new life without any crime or assassinations, ie a normal life.

2-I thought I saw Umaro on the ship at the end, but the reason you don't see Gogo is that he probably died from that fall.

3-?????

4-The world was slowly revivifying. Rome wasn't built in a day, you know.

5-Kefka was very easy. FFV's final boss was the hardest, imo.