Follow up to yesterday morning:
I tried again yesterday afternoon, and log-in went seamlessly. I slipped straight into the FTP test and rode for a total of 90 minutes, including the est and after. While there was some color distortion during the test, the nuts and bolts of it all worked as I expected it to. The access issues earlier in the day seems to have disappeared spontaneously, and I am left wondering whether it was a glitch in the app that the developers fixed, or whether it was a volume problem that overwhelmed the servers.
Reliability of access will remain a concern going forward, but the FTP test will be more useful to me than anything achievable on the road (uniform warm up, no stop signs, potholes, or dogs in the road) and so my feelings evolve into Cautious Optimism.
Many thanks developers and keep on developing!