Most agree that the solvent has changed over the years, but how do you know what it is?

Good to know about not using heat on the glue! I remember a guy who used a hair drier while patching piles of tubes at home.

WRT contamination from the surface of the plastic, any micro-layer of mold-release would not remain as a detrimental microscopic film, but would be miscible in the wet glue thus preventing it from having any significant effect on bond strength. Solvent-based glues are very good at dispersing any contamination "film" as long as the glue is agitated on the surface, and in any case the plastic wrap is not left between the tube and the patch. The plastic protects your finger while preventing skin detritus, moisture and oils from contaminating the glue.

You have to spread the glue around, else the glue would be dripping off the tube and the drying time would be enormous. Most patchers have noticed this I'm sure, that an even layer dries fastest.

Most tubes take well to patches and things become not so critical except for allowing the glue to dry, using fresh glue and not touching the patch. I typically use just a quarter of a patch for typical thorn punctures with lifetime reliability, but I do test the first patch that I put on any tube.