If we want to open a gate, the best thing to do is to ask Hyric which one is the less dangerous for the city to open. And if I understand well your plan, groupe A - the Runners, will be yourself, Galahad and Doogie on horses, maybe with two more horses. While group B - the Flies, Bunny and me, are the diversion. I don't have his destruction capabilities but I can fly and get their attention.
We never discuss about the sea way. The pirates are in the open seas, so I suppose we can maybe follow the coast line and bypass the horde. We can maybe make a deal with a captain brave enough to bring us there and wait for us and the supplies. Or he can go back in Waterdeep and wait for our signal. I have a way to send a message. Just my thoughts.