Yes two fighters of equal talent the one who is truly willing to die to win has a huge advantage. When you are in a real life and death fight which I have been in and a lot of other fighters have been as well you know the difference. Look no fight is worth your life I know that now but when you are a fighter your life belongs to the ring. You can't beat a guy who is willing to really die for it .It would be like fighting a animal they don't know that they are mortally wounded.

It depends on the fighter some fighters were born to fight and can take punishment but some fighters can't take punishment usually end up dead in the ring but it doesn't mean you shouldn't develop defense which is an important aspect of the game but to answer your question if their willing to die in the ring then they probably want the win more and can comeback in the later rounds.

As long as they live they could have an advantage because they are more willing to go all out. At the same time they can be at a disadvantage because they are more likely to suffer career ending injuries or even brain damage.

No because boxing isn't about who can take the most punishment or who is willing to put their life on the line. You can go into the ring prepared to die but that's not going to help you win.

Not later in life when they are in a wheel chair and can't walk because of brain damage.