1) You can vote for any delegate you like, up to 110 of them
2) The 101 delegates that produce blocks are the ones with the most votes
3) They are in charge until they are voted out of the 101. For each round of 101 blocks the order of the delegate block production slots is randomized.
4) Anyone can setup a delegate node, you only need to pay a registration fee equal to 2 weeks of pay at your desired pay rate. It's about 59k BTS I think at 100% payrate.