One last dumb question: can I split a PNR after having flown one of the legs on it?

Yes.

One other thing to be aware of - if there is a high likelihood of weather disruption on your flights, you may want to think twice about splitting PNRs. It resulted in a lot of additional phone time for me in the recent snowstorm as the agents had to do everything twice - and I was told that there would be no guarantee of us automatically being protected on the same alternate flight in the event of a cancellation.

The agent(s) could find the other booking easily enough each time I called, by looking at the notes in the original one from when they were split, but it certainly resulted in some more work.