Yes, that sounds like an interesting idea. Propane (or butane, for butane torches) is heavier than air, so it does displace it, and its oxygen, from the tin. That may avoid indeed the formation of the skin on oil paints. I guess also that being a hydrocarbon there is some affinity with oil paints acting somehow like a solvent.

That, plus inverting the tin for better sealing should make them indeed last for much longer.

In my experience, if the tin has been kept inverted and it is stirred well the paint is usable for several years. However, most manufacturers mention one year of shelf life if unopened (which implies much shorter if used). But this is not unexpected if they want to sell more.