Doing a little detective work:

Paverpol:  aqueous dispersion of vinylacetate homopolymer.  The mfg. says it won't melt polystyrene foam (styrofoam), so it must be lacking in any serious solvents.

Plaid Stiffy:  since it is non-hazardous, they don't have to identify it, but the same ingredient is in Mod Podge (often used for decoupage, dries clear).

Aleene's Fabric Stiffener:  non-hazardous, considered a trade secret, but the same ingredient is in acrylic paints.

Elmer's Craft Bond Fabric Stiffener:  Acrylic latex compound.

All of them seem to have the same base, polyvinyl acetate, or vinyl polymer, known in PM circles as PVA.  Each of them differ a bit with an added ingredient or two, but it appears to all boil down to "trade secret" names for plain old white craft glue. 

I would also be willing to bet at least 25¢ that the only real difference between any of the above and plain white craft glue is the amount of water in it, the fabric stiffeners having less, the white craft glues having more.  Even among white craft glues, the cheaper ones appear to have more water and less glue.

Personally, I would make a trial experiment and use the cheapest fabric stiffener I could find, and see if the results would be adequate for my needs.  And if I could find a little container of the expensive type, I would try that, too (with a similar test material, so you're making a reasonable comparison).

And then use a good white craft glue and compare that with the stiffeners and see if there's any difference.

Sue