Haha nice indeed.
For years, my sole source of parts for everything I home brewed was from donor boards... then a brief window where there was some new part purchases going on, but the last few years, I've been right back at it again... PC power supplies, Car audio amps, UPSs, cheap MSW inverters, among a few others are absolute gold mines for experimentation parts... and as you pointed out with the UPS/battery thing, they have not only typical but also usually fairly obvious failures, so you almost know what you've got in hand before you ever even remove the first screw to take a closer look
One thing... on paralleling diodes... it's actually very common in the SMPS world, particularly the TO220/247 Schottky "KAK packs" found in higher current lower voltage supplies (such as PC supplies)... The main thing is to balance each side with the traces they're connected to as closely as possible... and while that doesn't account for the minor internal differences, it's not generally a problem if they're properly sunk and not pushed all the way to the datasheet under warmer conditions. They're pretty tough little critters when they're done right.
Good post, glad to see I'm not the only one with projects like this that somehow just kinda stick to the wall/bench/table/ceiling haha... Efficiency is always a good thing, but there's a lot to be said for things that "just work"
Steve