These are more questions about magic in general;

Ok, I'll try and get all your questions, but some feature my own theories.

-Possibly because not everything can be fixed by magic. The human eye isa very complex organ. If even one thing went astray duringtghe process, tghe patient couldlose all vision.
-They never knew how. Their Transfiguration classessnever covered that sort of thingg.
-Thed prophecy. Also, he might have run into Harry and/or Hermione.
-Prrobably not. There was a huge expplosion caused by tghe event. Thechouse was blow tobits. Voldy's body didn't stand a chance.

Maybe magic is kind of like medicine and they haven't gotten that far along. Maybe it's too difficult or expensive to do and they don't want to risk it.

Because she never thought about it.

Dumbledore always thought that time traveling was dangerous and surely considered your idea but thought it best not to do so because it might change the way things go. After all, this is all about fate. Not about manipulation.

Voldemort was never found. He disappeared.