Prusa has some good info on it. It’s not recommended for outdoor use although I have some outside now doing fine so far (I’m testing PLA vs PETG). https://help.prusa3d.com/article/pla_2062

What does the oxygen double bond bit do? (Automation/controls here and high school organic chemistry was a long time ago, but I've printed kilos and kilos of PLA parts... some parts seem fine after being installed outdoors in full sunlight in a hot environment, they're nonstructural and covered in dust though.)

I'm sorry if this is inaccurate, I'm pulling from orgo 10+ years ago, and I feel like an artist using feelings.

If you look into molecular orbital theory, the double bond causes overlap, which makes the bonds unstable. It can more easily jump off and create a new bond. If you want to see an even worse situation, look at Nitrogen triple bonds.

Further, the extra oxygen attached to the carbon makes the bond a bit polar since you have such a heavy electron clump on one side of the carbon.

I think you’re confusing unstable with energetics it’s certainly unstable to UV but you’d have to look at the kinetics and enthalpies of formation

When neighboring carbons are linked to oxygen, you get a less stable molecule.

That said, yeah, PLA is known to not be very stable, and that's part of the reason its biodegradable. It degrades into lactic-acid based large molecules that living beings have no problem on destroying. (But are toxic when you get several grams of it, so beware if you are doing strange chemical experiments with PLA.)

UV light absolutely destroys it, and PLA isn’t exactly stable. Indoors it seems fine.

That's just an ester bond. That's extremely common in biochemicals, including food.

PLA is just going to break down into lactic acid. Big deal.