As mentioned above, I’m usually discussing with @chris.adams on #sailfishos. We have a periodic meeting every Tuesday at 8 a.m. UTC. Next one will be on January, 12th. You’re welcomed to join if the time slot is convenient for you. You can find logs of the channel if you are interested.
The closed source parts are using the .well-known capability to discover the user CalDAV path. But as far as I can see, this is done later on in the code path, after validation of the authentication on the server path. Which is failing in your case.
After further thinking on the matter, I’m not convinced personally that authenticating on one of the service path is a good idea neither, since this path may not be provided and only guessed later with the .well-known method. Which leaves the question on how to validate the provided authentication information (login and password) opened. That’s where it’s interesting to discuss with Jolla people before running to coding a solution. So let’s keep this topic opened for suggestions and brainstorming up to the 12th. Thanks by the way for the tests on the Kolabnow instance for the .well-known method.