I finally got the bike put back together enough to fire it up and let it warm up. My temp guage is working! I followed the troubleshooting path from beginning to end and the only conclusion is that I had a faulty electrical contact somewhere. I took apart and cleaned every connector in both of the groups that are above the fan shrouds. It's amazing how filthy and corroded they were! So faulty electrical connections knocked out my stereo info display on the lcd screen and my temp guage. Both now work!
As for the coolant leak...more amateur, anticlimactic stuff. I tried to plug the hoses that connect to the carbs with 3/8" wooden dowels when I did the pressure test. The right-hand one leaked a little bit. That was the source of the moisture that I found. I dried everything off and plugged the hoses with a more suitable object. I pumped it up to 15 lbs. and spent 30 minutes inspecting every connection. No leaks. My only conclusion is that one of the carb hoses was the source of my original leak. I will reserve final judgement and hang on to the pressure tester until I can ride it several miles.
I feel like I should get a certificate of merit for removing and replacing the fan shrouds (again). If there's a trick I haven't discovered it, but I'm getting better. I reinstalled both the mirrors today in half an hour and I'm proud of that, too!