Depends on how you think about each situation that happens to you. Do you take it as a lesson or do you toss it off as nothing more than a mistake you THINK you could never make again? (rhetorical)
Not really, if you break your arm it doesn't get stronger. Emotional trauma also doesn't make someone stronger, unless they learn from the bad experience that brought it. In that case, it actually would make them wiser not stronger.
It's then called experience and that is better than before. If you are really stronger (now using a wheelchair for example, depends on ones personality)... Some take even that as a challenge and not as a defeat
Yes. That's how immunization, especially "natural" immunization, works. So long as you have time to recover from it, you can resist it better next time.
You do gain unexpected strengths from your life trials though. Many times these new strengths go unrecognized.
PS: Not always.