1. If possible, I'd sort of like to do 2P Dominion. Do you think I would run into any resistance if I said 2P games only for my sessions?
Well, for the pre-tournament tutorial, my preference would be for 3-player, to prepare people for the tournament. That said, it's your event, so you can run it however you like. You could adapt on the fly, based on what the people who show up want to do; you could get a family looking to learn the game, or a set of cutthroat MtG players looking to duel each other.
2. I'll be bringing my set, which can support up to 6 people at once if we do 2P games, and 9 people if we do 3P games. If we need to accommodate more people, then I'll need more sets of base cards and using sleeves becomes a little more tricky. Will this be an issue? Would it be better if I just say "my event can only support up to this many people" or to say "I'll need materials if I want to go over X people"?
When you register your event, you can list a maximum and minimum number of players.
3. Do you think a strategy tutorial would be more popular, or should I just teach and play the game? Or do I even get that option? Would it just be "Dominion with Adam, a really good player who can help you improve no matter what your skill", or "hey come learn to play Dominion, Adam will teach you the rules and guide you through a game"? If I get a choice, I'd prefer to not be teaching mechanics the whole time and actually get into some interesting strategic concepts, but do I even get that choice?
You do; it's your event, and you can run it as you please; you'll be submitting your own description for it when you register. My suggestion would be to register three events, each taking one slot (about 2 hrs); this adds up to the 6 hours of game-running needed to qualify to get your badge refund:
1) "Introduction to Dominion" - On Friday, for rank newbies, maybe with the "First Game" Kingdom.
2) "Advanced Dominion Strategy" - Friday night or Saturday morning; assume you've got veteran players, and can go deep strategy.
3) "Tournament Redux" - Sunday; you let people play (or replay) the tournament sets, to give them the chance to try any they didn't get to see, or to test their strategies against each other.
One note from experience: There's a fair chance that you could get a table with large experience disparities between the players. In such situations, I usually handicap the stronger players by swapping out some number of Coppers for Curses in their starting decks. One is a mild handicap, two a strong handicap, and three a severe one. When I'm filling a seat in a teaching game with newbies, I usually take a three-curse-swap handicap, just to make sure I don't win.