WD-40 is mostly solvent with a little oil in it, so it works great for a week and then the solvent evaporates and leaves behind a sticky film that grabs every bit of dust in the air. I read a post on a lock picking forum a while back from a guy who's been a locksmith for 30 years, and he said the same thing. He refuses to touch the blue can on anything with moving pins. What he uses instead is plain old dry graphite or a Teflon based dry lube like Tri-Flow. The key word there is dry. You want something that goes on wet enough to get in there and then dries and leaves a slick coat behind, not oil that stays tacky. On a 71 Nova trunk latch you could also just hit it with a tiny bit of white lithium grease on the moving parts, but keep that stuff out of the actual key cylinder or you'll be right back where you started.

Honestly jades are tough as nails and I've watered mine with straight tap for like 8 years, ppm probably close to yours, and it's fine. Leaf drop in November is way more likely just the plant going dormant plus less light, that's totally normal for jades that time of year. The distilled water fix might just be a coincidence with the season changing back, but hey if it's working for you then keep doing it, your mileage may vary.