No reply from Red Devil River after a week. New Ebay-sourced cable from recommended vendor arrived tonight. I hooked it up to my old Windows 7 Dell (pretty much just used for flashing my vehicles) and the cable automatically loaded the FTDI driver from 2017, which is probably the one that I had previously loaded while troubleshooting the RDR cable. So I tried flashing and..
IT WORKED ON THE FIRST TRY!
So, yeah, the RDR cable apparently is junk (it seems to read fine, just not good for writing.. ).
--'94 Caprice "SS" wagon, 0.30 over, high-volume oil pump, LT4 hot cam, 1.6:1 rockers, ported aluminum Corvette heads, shorty headers, PCM4Less tune, 4.10s, shift kit, side-exit dual exhaust with Flowmasters, OR X-pipe, F-body gauge cluster, Bonneville front and rear seats (working electrics), Bravada console shifter, Bilsteins, 12.7:1 Impala steering box, Boss 338s with 255/45R20s, bunch of other stuff I'm forgetting.
--'04 Mustang Cobra, '15 Audi S4
--1125R, 1050, VMax, VFR800, DRZ-400E