I’ve figured out my citation woes…at least for the moment. Don’t use the “url” type in bibdesk. Use “electronic” instead, which is supported by biblatex. Then, tell bibdesk to include a “date” field by default. “date” is the date that is on the url, “urldate” is the date you visit the url. Then everything works as it should.
But it certainly is confusing, and trying to figure it out took me deep into the depths of latex hell, something that I don’t want to do again anytime soon…