Damage that is redirected is not dealt to the original creature or player, but it is dealt to the new creature or player.

You only gain the life when the triggered ability resolves. If you are reduced to zero life before the ability resolves, you will lose before gaining the life.

You gain life for all damage which is not prevented, regardless of what the creature damages (player or another creature) or the toughness of the blocking creature. If you put this on a 5/5 and it is blocked by a 1/1, you still gain 5 life.