Update, or lack thereof:
I pulled all the afflicted plants. 3/4 did have damage to the stem about an inch and a half under the soil line. Didn't look chewed on but more burnt: discolored, darkened, and kinda "turned down... " also, all the plants had way less roots than I'd expected to see, almost like the roots hadn't grown since I transplanted from Solo cups... Not sure if this means they got ate or if that just failed to thrive but the leaves, trunk, and branches had all grown, sooooo... The 4th plant has a normal looking stem and the roots were still less expansive than I'd have expected, but the rootball was bigger than the other 3. Fwiw, that plant was the last to show symptoms... Maybe I jumped the gun?
I saw no bugs at all among the roots and immediately surrounding dirt, except a small centipede just outside the one plant's rootball. I looked pretty hard, too.
Only been a few days but the rest of the plants aren't suffering the same symptoms, although I do feel nervous as hell everytime I check my plot. Hopefully, predators and/or metamorphosis have removed the threat...
Still got other problems. Some of my Chinense plants have the whole spinach leaf thing going on. I am thinking it's a calcium problem? Also having flowers drop on other plants but I think that's more down to the heatwave than anything else...
Overall, I remain at least halfway confident, as I have a lot of pods setting and some annuums are even coming ripe. I'm also pretty danged humbled; I still have much to learn about growing these plants...