The general rule is that the strength comes from the glass fibers, not from the resin. All the resin has to do is stick the fibers tight to the surface, and when the part is stressed, it's the fibers that do all the resisting. I've used both polycrylic and epoxy for finishing sheeted parts, and can see advantages either way. I tend to use epoxy, mostly because I have better confidence in primers that are formulated to bond to epoxy. But I've been happy with my polycrylic jobs as well, after adding a few finish coats. Fast drying time is a major advantage, as is water clean-up.

I'm not familiar with the Elmer's product, but thick viscosity sounds like it would not be so helpful. Experienced glassers use laminating or finishing formulations, which have a fairly thin consistency. I tend to use West products, which are also fairly thin, and have a variety of hardeners (fast, medium, slow cure). I buy it by the gallon. Shelf life is pretty good--some of my resin is at least five years old, and still good.

Thinning with alcohol is normally aimed at spreading the epoxy in a very thin coating, thick enough to bond the glass cloth to the surface, but thin enough to keep the weight down. Epoxy can add a lot of deadweight if you're not careful. But where you're making up a piece entirely of fiber/epoxy with no substrate, you need enough epoxy to fill the weave of the full laminated thickness. In making up retract doors, I'd think thinning with alcohol is not the way to go.

The best way to bond additional layers of glass is to do all the lay-up while the resin is still semi-liquid, before it starts to set. Start by coating the surface with epoxy, then lay the cloth down on the wet surface and stipple it down with the point of the brush, or with a fine-textured paint roller. Make sure you drive out any air bubbles under the cloth. Then when the first layer is down, add the second, then the third, stippling or rolling each one down into wet resin. Obviously you want a slow curing hardener for this kind of work, which is typical of laminating & finishing products.