Plot details to Harry Potter and the
Half Blood Prince ahead. If you haven't
read it yet and don't want to know what
happens, stop reading this post now.
The first question, if you have been reading the books, is, to be blunt, dumb. By the end of Harry Potter and the Half Blood Prince, Ron and Hermione are together as a couple and Harry is breaking up with Ginny, Ron's sister, so that he can go off unencumbered and fight Voldemort. I would say that it is clearly more likely that Voldemort kills all the heroes and emerges victorious that for Harry to wed Hermione.
But the second question is actually interesting, but only because I think that it is pretty unclear still exactly what side Snape is really on, so it isn't clear what it means to "turn into a 'Professor Snape'. I think that despite the fact that he kills Dumbledore at the end of HPAHBL, he is still working undercover for the Order of the Phoenix. In fact, I believe that if you read carefully, the way that J.K. Rowling has written both the killing of Dumbledore and Snape's escape afterwards, it is ambiguous what is going on, though it appears to Harry (and the reader, at first) that he has betrayed Dumbledore.
Voldemort has assigned the task of killing Dumbledore to Draco Malfoy, Harry's classmate. Draco finally has Dumbledore cornered and disarmed, and Harry is invisible under his cloak and frozen, unable to help. Yet Draco cannot bring himself to kill Dumbledore. Dumbledore talks calmly to Draco and urges him to come over to the good side, that he and his family will be protected. Then four adult Death Eaters (followers of Voldemort) show up, and Dumbledore continues to talk to them normally. Then Snape arrives.
"We've got a problem, Snape... the boy doesn't seem able --"
But somebody else had spoken Snape's name, quite softly.
The sound frightened Harry... For the first time, Dumbledore was pleading.
Snape gazed for a moment at Dumbledore, and there was revulsion and hatred etched in the harsh lines of his face.
Snape raised his wand and pointed it directly at Dumbledore.
and Dumbledore is killed.
Notice that Dumbledore does not plead with anyone except Snape (indeed, he mocks the adult Death Eaters), but Rowling doesn't say that he is pleading for his life. My theory is that he is pleading for Snape to kill him so that Snape will be able to continue his work for the good guys. In fact, Snape doesn't kill Dumbledore until Dumbledore pleads a second time.
Then, when Snape is escaping after killing Dumbledore, Snape stops Harry from using several spells, including an Unforgivable Curse against him. Now you could say that he is only protecting himself, which is true, but he also tells Harry "No Unforgivable Curses from you Potter!" By stopping Harry from using such spells, he is also protecting Harry, since using those spells is, well, "Unforgivable". Moreover, Snape doesn't get upset about Harry trying to attack him with these spells, but when Harry calls him a coward...
"DON'T --" screamed Snape, and his face was suddenly demented, inhuman, as though he was in as much pain as the yelping, howling dog stuck in the burning house behind them -- "CALL ME COWARD!"
If Snape is playing this double (or is it a triple?) game, then he is taking enormous risks. Moreover, he has just unwillingly killed the one man who has always trusted and supported him because that is what. Having Harry call him a coward is too much for him to handle at this moment, which is why he snaps.
(I call this "my theory", but I would guess others have made this guess too.)
So, maybe Ron will "turn into a 'Professor Snape'".