I'm new to this, and any help would be appreciated.

BACKGROUND: I'm a student that only spends about $1,500 per month, and therefore I'm looking to rack up as many miles as I can through the initial sign up bonuses of credit cards.

QUESTIONS:
(1) What do "targeted" credit card offers mean, and how do they differ from non-targeted?
(2) I signed up for a Chase United Mileage Plus Explorer card and got the 30k bonus miles. Is there any way to cancel this card and sign up for it again to get the bonus again? (it says the bonus is not eligible for prior or existing members.)
(3) If I wanted to continue getting UNITED miles, what other cards / offers give united bonus miles, or points that will transfer to united miles? (I'm looking for large (20k+) initial bonus mile awards, as I don't spend much money per month.)
(4) Are there any cards that you can sign up for, get the initial bonus amount, then cancel the card and do it again?
(5) Which airline has the best frequent flyer program, in terms of getting the most miles the fastest without spending much per month on a credit card?

Thanks!