A lot of people like to point out that Harry Potter grossed more than Twilight movies, but don't these factors play a huge part in that?

1. As far as I know the price are more or less the same. Even if there is difference mostly it got something to do with the building thetre management rather than WB/summit itself (ex: the price is slightly cheaper in thetre A than theatre B because B is more exclusive place to go, etc) even if there is difference I don't think it's that big. People won't buy it.
2. Have no idea, but again. Promoting internationaly is easy, since mostly if it's popular in america it will be known internationally. And even if say, some country didn't show it in theatre, people/fan can still get a hand on it via dvd. So unless it's some backward/wartorn country I think people everywhere can still get their hand on both movie equally.

Not really, unless one movie was actively negatively  targeted. Ie if more countries refused to allow Twilight to play in their theatres than HP or if Twilight had been forced to charge th prices they did. Otherwise, all things being equal, it simply means Twilight movie makers just made the wrong decisions. The same way decisions made on the script, casting, special effects, etc impact on how successful a movie is, so do decisions on the marketing of the movies. Marketing is just as if not the most important part of movie making. Ticket  prices, how, when and where the movie is promoted have a huge effect on ticket sales. It doesn't matter how brilliant a movie is, if the marketing is such that it puts the movie out of reach of the audience, then it's obviously not going to do as well, and they have no one but themselves to blame for that.

Second guessing after the fact does not change anything. Would H P have been even more successful if they had cast child actor who'd have grown up to be taller and better looking than Daniel Radcliffe? Hindsight is 20/20.