Final report, after the shop replaced the 80/90 gear lube with the Miata proprietary tranny lube, which turns out to be a 35w/90 syn product. Never knew such a thing existed...
I drove it quite a bit yesterday, and the problem of notchy shifting in 1st & second is definitely gone. Getting "locked out" of 1, 3 & 5 was a one-time event that never recurred, but I'd bet it too was related to the too-heavy oil impeding the function of the synchros.
So: if driving an ND, be sure the get the "real stuff" from Miata if you're refreshing the tranny lube...