1. From my experience leveling goes slower the more people you have in the party. When you begin to exceed dualboxing collection quests start to take more time than they are worth. They would be decent if you didn't have a group penalty for single mob kill experience, but you do so it sucks. Triboxing collection quests is manageable, but then you can't really do instancing without waiting for 2 more people to join up and then you have to hope they know what they are doing . Look at the bright side though, with recruit a friend I took a group of 5 from 1-15 in 52minutes without trying very hard at all. With this new feature leveling is just plain fast.

2. Paladin make the better multiboxer tank by popular opinion, and I don't disagree having experience with my well geared 70 paladin.

3. Shaman are decent healers, but chain heal isn't something you want to spam/use when you have a pally tank. A shaman will work fine, but I'd say a priest with Circle of healing and strong single target heals is best for a pally tank. You can't go wrong with either, but don't forget druids are also getting a great AoE bloom spell in WoTLK. Really, any healer will do, it's just personal preference.

4. No, shadow priests do not produce much mana at all in a 5man. That more of a raid utility. For the short term pulls you'll do in a 5man a warlock, mage, shaman, ect will not run out of mana during a pull unless you're spamming something you shouldn't be. Plus, it takes a lot of micromanagement to stay under the threat level and still produce decent dps as a shadow priest. If you're up for the challenge, then give it a shot, but yah the mana production isn't a decent reason for taking a spriest to a 5man. If you are just looking for what would work best, go for synergy. Shadow priest + warlock, or fire warlock + fire mage, or 2 frost mages for easymode, or moonkin + mage or shaman, ect. Look for stacking buffs like scorch and curse of elements and such.

5. I'd say no. Looks like it could manage 2boxing pretty well, but 3 boxing might really strain that system. A lot of people ask this same qeustion when all you have to do for the answer is create some trial accounts and see for yourself.

6.Good luck!