June updates:
I ended up ordering some seed from Sheffield's, which said it was collected in Paradox which is pretty nearby. So far, only one of those seeds has come up, one that I direct sowed into a lightly mulched bed of native soil, a couple weeks before last frost. It took quite a while to pop up, so I was probably too early with that. I don't know if the other seeds in the same bed are still dormant, or got eaten by birds, or what. The seeds have a really hard coat on them so I doubt that they've rotted already. The seedling is a very generic looking monocot, growing very slowly, and I might have mistaken it for a weed or not even noticed it was there, if I hadn't marked the spot where I planted it.
I also took another cutting and put it in a pot with some "cactus mix" miracle grow stuff, and this time I've been watering it about as often as any garden plant. I read so many warnings about not overwatering cactus, and I'm sure that's a real problem when growing them in wetter climates, but I guess I don't need to worry about it too much in this case. I had it in full sun at first, and it very clearly got sunburned, so now it's in afternoon shade and seems to be doing alright. It's definitely still alive, not drying out and turning grey/brown like the others did, but the leaves don't appear to be growing at all. I hope that means it's putting energy into growing roots.
And finally... it seems that whatever is eating the wild ones isn't deer. Or at least, not exclusively deer. I put a circle of hardware cloth around one that was flowering, and I guarantee there's no way a deer got around that, but the whole flower stalk still got eaten. So it must have been a rodent or a rabbit, both of which could easily get under the fence that was only meant to stop deer. So the next time I try that I'll have to actually fence the thing all the way down to the ground.