1. Can i kill Masterming w trigun w|o invulnerability and proceed to next lvl?
- Killing the mastermind with trigun normal bullets will get you a normal ending
Killing the mastermind with trigun angel arm, when not invulnerable, is exactly the same as any other method of nuking yourself - you'll be dead, and will get a partial win, ergo, you will NOT proceed to the next level.
Your other 2 questions are pretty self explanatory. I get the impression you thought angel arm only did 5hp damage to you when you are not invulnerable, and it doesn't work like that... It removes the 5hp *IF* you are invulnerable and survive the nuke.