Woo hoo! I figured it out.

There were actually 2 separate problems. My spokes really were about 2mm short. Normally I would have been able to lace the wheel with spokes that short, but I also had the pattern, relative to the 2 flanges, off by one hole. Each problem kept me from being able to see the other.

The nice thing about a problem that stumps you is how good you feel when you finally figure it out. The more frustrating the problem, the better you feel after solving it. I'm feeling pretty good this afternoon.

I hope it's warm next weekend.