1 - Why? Roles are agnostic. Neither tanks or healers are actually tanks or healers and PLD is the only one that is actually "tanky."
2 - The UI tells you if someone has LB or not. Bait the NIN and keep your HP above 50%. Its not difficult to make them waste it. SAM is literally as simple as marking them with the do not attack marker and then just ignoring them or stunning them and kill them.
3 - Why?
4 - Why?