I'd imagine that the possible deterioration in the 35mm is due to the thorium yellowing the glass. It's also a problem with some Pentax screw mount lenses. It can apparently be fixed with exposure to strong UV light - your best bet is probably to build a lightbox with a UV light as that'll be much more reliable than trying to keep it in direct sunlight (a lot of the UV in sunlight is blocked by window glass anyway).

The major problem with the "buy a Leica" mindset is cost and availability. A Leica M series worth owning seems to go for anything between 300 and 1000 here, some of those don't include a lens. I refuse to believe for one moment that they're six times as good as my Pentax MX and 50mm f1.7 despite being six times the price (at least!)