Update: FINALLY got down to the range with the CM40. Put ~100 rounds through it and had only one jam. (And it was a more conventional 'jam', where the bullet got fed into the chamber at the wrong angle, not what I was seeing before with the slide misbehaving.)

It's still very frustrating to get a gun that was so obviously wrong straight from the factory, especially when it sounds like I wasn't the first person to have this issue. But Kahr handled it pretty well IMO. Sent me an RMA same-day, paid for overnight shipping both ways, had it back to me within the three-week period specified on their service guidelines, replaced the barrel and guide rod along with the spring (I'd imagine it WAS the spring, but of course who knows).

I'm going to consider this case closed...