The side effect of Sylar's power, this "hunger" is always on. Sylar can and does manage it but Peter needs to learn.
Because of the differences in intelligence, will power and temperament, I think Peter is going to have a fundamental problem learning to control the hunger, moreso than Sylar.
And I haven't interpreted Peter's ineptitude with his powers as anything more than the writers conveniently forgetting that he has some power that could end the scene in two seconds flat when they need the scene to go on for ten minutes.
On another topic, I
do think we'll see Vortex Man back - because in the hands of someone who couldn't control it well or who had malicious intent (such as Peter), a vortex could be the thing that tears apart Earth...I guess it would stop there and not swallow the universe because the weilder would die of suffocation when the atmosphere disperses. Even someone who couldn't die would pass out from lack of oxygen(?)
They haven't. But they also haven't tried doing it when the other was unaware of the attempt. Every time it has been obvious to the other with both of them trying to read the other's mind.
That's what causes the feedback. You can't have feedback if they're not hearing the other hearing them hearing themselves hearing the other as they try to hear what the other is hearing... and great, now I have that shocked look on my face that they get.
The feedback could also happen passively, even when they aren't trying to read the others' mind. We don't know whether this is the case because like you said, neither has tried it when the other was unaware, so the rules of the game are still unclear. I think storywise it would be more interesting if Peter and Matt were mutually immune. The writers will probably opt for whatever logic fits the story best, because that's their modus operandi so far.
Quick question that has bugged me since the beginning of the season. Peter can heal. Why would Future Peter have a scar?
When Claire was cutting him with a knife, he didn't heal immediately because the Haitian was in the room. Presumably even when he got away from there, he should have still had the scars. However, I don't recall him having the scars in his later shirtless scenes (perhaps I was too distracted to notice

). That would have been a nice detail for the writers to throw in. Anyway, in any future timeline when Peter gets cut on his face around the Haitian (by Claire?!?), he will have the scar.
I don't know if it's logical that he doesn't heal after he's away from the Haitian, but I'll give em that one as a freebie. It's not obviously illlogical, at least not by
Heroes standards.