I thought this was a marvellous puzzle. Difficult, certainly, but most ingenious. I am hesitant about the penultimate step. The preamble says that the side to move next is determinable from the gridfill. This is a pretty broad hint, but I don't see it
More worryingly, I can't get the entry for b4 to work. If I am correct, it must be occupied, but I need one of the squares it can move to to be a capture on a3, and that is unclued, which suggests it must be empty.
Everything else works, so i feel I can't be far away, but it's niggling me.