Bummed out by the fact that Quill wasn't enabling me to syndicate directly to Twitter, I followed up on some good advice from Daniel Gold: Back to basics, uninstall and reinstall plugins one by one. Shades of WordPress. So I did that, and here's what I found:
With IndieSyndicate configured (*via* silo.pub) I can posts to Twitter just fine, but Quill still does not see that as a Syndication target and Quill cannot post to my site.
In retrospect, that's obvious, because there is no endpoint at my site.
So I enabled IndiePub and now Quill posts fine, but it still does not see any Syndication target.
I probably just have to live with that. At least for now.
Finally, re-enabled Brid.gy and everything looks good once again.
Just for the record, here, I've decided that for now I do not need these plugins: Static pages, Firefox, Events, Custom JS, Custom CSS, Comics, Audio, API tester. That may change in time.