Well said, I get the emotional part of down pact, but the physical is still a blank. Trying to apply logic to it won't clear it up, logically Tsunade shouldn't or couldn't have healed herself the way she was, technically she should have been dead. Who knows, maybe her working on Lee - may have unlocked hidden truths to a new regenerative technique that she used on herself, that can heal nerves, bones, muscles and all. Mind frame wise, we could apply that to Obito who was prepared to die once he threw Kakashi away before being crushed, half his body was crushed, along with with half his skull, just as much never ending damage.
But, maybe Gates not only intensifies attacks, but damage also, that would be a clear bell.