Time travel questions.

Ok so I just got done reading Harry Potter and the Prisoner of Azkaban and its left me thinking about time travel a lot.

The first question is regarding the book directly though. SUPA SPOILA

Anywho, so , in the end when Sirius and Harry are surrounded by all the dementors and were about to both receive the kiss. Then they were saved by a patronus that was believed at the time to be James but is later discovered to just have been Harry. The problem I’m having with this is that some version of Harry would have had to survive the scenario to be able to make it to the hospital and use the time turner to go back the very first time. Which, seems to be impossible giving the scenario they were facing. Is this just a plot hole?

Second time travel question. If one were to go back in time 4 years to visit an earlier version of himself. He doesnt interact with that past self in any way just observes them. Now on this way back to his time machine he is killed by a stray <insert random noun> . Would that be the end of that mans life? I mean the younger version would still live on to grow to that fatal mistake right? I really dont know where im going with this I have a lot of questions about it but not sure how to really word it. egh >_<