Also, I noticed visible differences between supports treated differently (more or less sizing, one or two coats, more or less formalin). Hot-dog rolling leaves a relatively rough texture, which I am concerned might create micro-bubbles or weaker points during mating. So even if up to 10% concentration is suggested for gelatin, I prefer two coatings of 4% and roll until the gelatin starts to set and yields a finer texture

Have you tried pouring a small amount on the support and coating with a copper pipe? You end up with a nice, smooth surface. Smooth surfaces seem to work better.
I size my papers with acrylic medium (hot dog roller). They have a slightly rough surface when dry, but when I zap it in a dry mount pres,. I end up with a very nice, smooth surface.