We sailed on the Fantasy in December. The ship left Port Canaveral hours behind schedule because, as the captain said over the intercom, "We're waiting for some late flights to come in." This was when there was a winter storm up north. I assume that the people the ship waited on purchased the flights via DCL, so it seems very likely they'd hold the ship a while at least if some DCL-booked flights are late. No guarantees but it happened on our cruise.
I read a year or so back a post by a woman who purchased flights & insurance via DCL, but the flights were so late that even though the ship waited, it finally sailed without her. DCL did fly her to the next port to meet up with the ship, but she was still steamed.
Moral of these stories, they'll get you on the ship eventually, but if there are delays, the experience might not be as smooth as you'd like. I have a high tolerance for risk, so in your situation, I'd personally book the flights & insurance via DCL & just go for it.