Back from my trip, and everything went perfectly well. (I even got an op-up on the IAD-LHR leg!)

UA couldn't print boarding passes for SK, and SK couldn't print boarding passes for UA. That was really the only minor glitch -- it just meant another chore for me at LHR chatting with a friendly agent to get my passes.

I needed to check in for the return flight at ARN with the SK conf #2. But all the flights were on time, and I didn't lose any luggage, so I'm happy!

Thank you all!