Hmm... that could work, and the battle would take up enough time for the castle to get to Tar Cantun.
You have a scene where alain tells (rants at) the "Arrogant noble idiot" about how its unlikely that the city will actually be preserved then we have a fight with Engo that takes up time, after he dies or flees we can have a scene where we arrive at the port, we see the castle floating towards the island, obviously way too far in the lead to overtake before arrival and I could have gilbert hold back thomas saying "it's too late". Then we could go from there.