1) You have the option of either using many controllers or only using one and passing it around.
2) In order to play multiplayer, you have to finish the single player mode. However, if you have an SD card, you can go over to wiisave.com, download a save file for Rayman, and transfer it to your Wii. To get to multiplayer, you just go into score mode. From what I've played so far, you have to select individual mini games to play them. I haven't seen any modes like a tournament mode that preselect games for you to play, so you might end up going around choosing a random game to do if you've never played Rayman before.
3) It isn't as parent friendly as Wii sports since the motions aren't as intuitive, but it's still not too hard to pick up and play after reading the directions before the mini game.