You seem to be going to a lot of effort to remove the anti-halation layer which should come off easily.
I have not used FP4+ in 5x4 but when I use it in 120, all of the dye comes out in the pre-wash. I can prove that by doing a second pre-wash from which the water comes out fairly clear.
It sounds like the problem is in getting enough flow of water around the back of the sheet where this layer is.
I don't think re-fixing is needed as the flow around the emulsion side is fine. Have you tried washing the dye off under a running tap or in a tray rather than in the tank?