the spanish control the fort now so they would have to retake the town. not sure why the spanish would stay though, they just wanna kill all the pirates and then leave. I don't think they know about MaroonTown so not sure how much longer they are supposed to hang around nassau.
I think billy saves madi and takes her hostage to try and find out where the urca cache is. the setup is right - billy is freed and leaves the underhill plantation, he is weak and wounded and will naturally go to the other place he considers "safe", the base he was using (flints old house). sees madi, grabs her and leaves.