Soooo... I was messing with the trigger, and noticed that the hammer didn't fully engage the trigger sear. It was offset toward one side. Further inspection revealed that the hammer itself was actually bent, as in curved on the plane where it should have been flat. Drove back to Briley today. The gunsmith who installs the triggers looked at it and agreed it was bent. Installed a new one while I waited. Alignment still isn't perfect, but it's not rubbing anything it shouldn't be. I didn't notice the stock trigger being all out of whack, but I didn't look at it as closely. The gunsmith said the stock ones are all over the place, too.
Threw the gun back together tonight. Haven't put it on the trigger scale, but it feels about right for weight. No takeup and no noticeable creep. Very crisp. Now to remember where I put that still-sealed Nordic extension kit.
Yesterday, I dropped off my defective M&P 340 with FedEx to go home and get fixed. I'm getting tired of this stuff, seriously.