Only
DCL transportation (and also any private carrier offering such a written guarantee) guarantees you will make your
Disney cruise, although in extreme cases of delay they will take care of you in the meantime and put you on the ship at the next port of call instead of at Canaveral.
I'm not sure whether one set of DCL transportation will cover a land-sea-land vacation. If not, then order the DCL transportation for the first part of the trip (with the non-stop from resort to seaport) and you will finish up th sea portion at the airport. Reserve and use Magical Express for the second land portion.
Taking Magical Express to the airport and renting a car from there will work fine if you allow plenty of time. I don't know how long it takes to get to the port so I don't know what time to shoot for. But don;t cut it close getting off the DME bus and over to the rental lot.