Actually, sodium carbonate will probably work for both. I *know* it will work for remjet; I've used it to take the remjet off a roll of Kodachrome I was trying to process as B&W (it was so fogged there was nothing visible on it, but the remjet came off nicely).

Just remember -- if you get any liquid on the emulsion side of the film, it'll show in the final image as a dark (after reversal) spot, because the water will wash out sensitizing dyes. I've seen this effect on film even from just a few drops of condensation.

To glue the two together -- could you print your screen on the gelatin side of fixed-out film, then dampen the gelatin and apply it to the back of your Fortepan? Gelatin is terrific glue. However -- you're back to having the film base thickness between the emulsion and the screen, which I recall you already verified to cause color shifts with veiwing angle...