Thanks for all the advice here. I have sent AC a complaint Email, but let me document the particular sequence of events that seemed to work for me:

1. Select change password from AC.com login page
2. Got redirected to a page hosted on AE.com where I changed my password
(curiously, the old password always worked fine on AE.com, even thru this AC.com snafu)
3. Make a trial booking and (finally) successfully login using new password via "back door" approach documented here
4. Go back to home page (effectively cancelling the trial booking)
5. Try to access my flight-pass and get yet another error message I hadn't seen before
6. Undaunted, browse around AC.com a bit more while still logged in
7. Second attempt at clicking on my flight-pass button works

There were a few more seemingly random glitches in making a real flight-pass reservation but some creative retrying of other itineraries finally let me complete a booking. What a mess!

If I wanted to play a web-based adventure game on my PC, I would have bought one - but this pathetic sequence of events is not my idea of an acceptable user experience for a seemingly modern e-business.

Hope this helps others in my situation (seems to be mostly SE's caught in this snare).

In the meantime, shame on AC's IT for this pathetic long-standing authentication outage, and shame on AC's management for not being more forthright about this serious issue!

Not only has AC lost several reservation fees as a result of this, but two cross-continental flights as well because Expedia offered routes on AA which were more convenient.

How long will this travesty last? How much more inconvenience are AC's top-tier customers who have already pre-paid for their flights supposed to tolerate?