The main advantage to question 1 is that if DCL is flying you in, they'll do everything they can to get you on the ship if your flights run late. On our December Fantasy cruise, the ship left Port Canaveral a few hours behind schedule for this reason (it was during a winter storm that had gummed up travel throughout the northern states).
In response to question 2, yes, they provide ground transfers to anyone who wants them (you don't have to book flights via DCL to book their transfers). You can also book a pre or post cruise hotel stay via DCL.
THIS>>>>>> Exactly! DCL KNOWS you are on a delayed flight if you book transfers through them. Best $35 per person I've ever spent!