... It forms a gooey whitish mass ...
Yes, that is the mess I get. Introducing Acetone into the mixture gives this mess every time but extra heat doesn't get it to dissolve. Adding methanol does give this mess too but to a lesser extent and additional acid will cause the mess to redissolve. Unfortunately, far too much acid is added and you'd nearly choke on acetic acid fumes. (Excess acid will negatively affect emulsion speed.)

My last tries yesterday eliminated the acetone. It seems any amount of acetone destroys the this Grafix acetate beyond usefulness.

However, adding melted gelatin to warm methanol seems to work. I can make subbing number 2, (the one Glafkides gives for nitrate) with no problem. Plain gelatin on acetate subbed with this mixture will adhere nicely. After drying I can't seem to peel it off by picking at a cut edge with a fingernail or by tearing the film. Will have to try the "scotch tape test" next time.

One concern I do have is that the methanol distorts the acetate as it evaporates. Not nearly as much as acetone does but a flat piece will not be flat but wrinkled after drying. The methanol causes the acetone to swell (which is the point) and maybe less methanol and more water should be used?

What might the difference be between using methanol and ethanol?

I wonder what the wet-media films are subbed with? Somewhere it was written that it is a latex coating. The Grafix PET films, as well as 3M and Dupont films, work the best as far as off the shelf items are concerned. I am guessing, though, that the Grafix product is really made by 3M or Dupont. (3M and Dupont only sell B2B in large quantity.)

Minwax polyurethane is a new one. Maybe it has potential.

( On a side note, since someone asked me about this, Glafkides books, "Photographic Chemistry" from the 1950s are generally expensive. If you author search under the man who translated these books from French you can sometimes find copies at a much better asking price. His last name was "Hornsby." Be careful, though, because Mr. Hornsby wrote his own book entitled "Basic Photographic Chemistry" at about the same time frame. )