1) Are rogue stars and planets created by galaxy collisions ?
Star yes, planets no.
2) If we replaced the Sun with another star during a collision with another galaxy, would we all die ?
Yes. Yes we would.
3) Do black holes convert energy into mass ?
4) Why do black holes bend space ?
Black holes hate space and are engaged in an ongoing war to destroy it. They hope that if they bend it enough they'll break it. The reason is that they had a big argument a few billion years ago (something to do with whether or not infinity is a physically meaningful concept) and neither side has yet been able to outwit the other.
5) If space was curved enough could I see the back of my own head ?
Yeah, but just use a mirror, OK ? Seriously, you can pick one up for like £1. Leave space alone.
6) How do we know space is curved ?
7) If a star dies , does it still have the same mass ?
8) If a star becomes a black hole does its mass increase ?
It can if it wants to.