Steve Beaty feels the Nemourids that I've been finding are most likely Prostoia completa. He does not feel the presence or absence of intercalary setae on the caudal filaments is a reliable character in distinguishing similis from completa. This is something he had already noted in his key ("The Plecoptera of North Carolina," p. 16): "Speciation between the three regional species [of Prostoia] hinges on the development of the intercalary setae on the cerci which has yet to be proven reliable."
More reliable, apparently, is the length and density of the silky setae on the tibiae: on completa they're long and dense (well developed), on similis short and sparse. He has sent me illustrations on the tibiae of both species that will be published in a forthcoming key developed by Stewart and Stark. The tibiae of P. completa look exactly like those on the nymphs that I've been finding. E.g.
In Beaty's 2015 key, he adds on P. completa: "This is a common species in the east and possibly in North Carolina. It occurs in small streams to large rivers (Kondratieff and Kirchner, 1984a).
So there you have it. Now we know what they are.