While troubleshooting my cold-start rough idle issue, I decided to test the WUR by disconnecting the electrical connector to the WUR. I then managed a start, then quickly went back to reconnect the WUR while monitoring the pressure.

With cold engine, and 72F ambient, my initial control pressure was 2.45 bar. After 30 seconds, I had 3.65 bar, with some continuing slow rising after that. No problem there that I can tell.

But then I wonder - if I had to crank excessively before achieving start-up, wouldn't the WUR have already leaned out the mixture?

I've seen similar issues with carburetor-ed engines with electrically-heated chokes - prolonged starting/cranking resets the choke before actual start-up.