OK, just got my car back from the Rocklin service center. They updated the software on the car: it's now version 5.0 (1.35.107), as opposed to the 5.0 (1.35.98) in the loaner.
The new software didn't fix the crazy nav system; for that, they completely replaced the MCU, or Media Control Unit, which most of us know as the big 17" touch screen. That fixed everything.
I asked my service advisor how many of these he's seen, and he said mine is the second. When I noted that I thought there would be more, he said he is only responsible for 500 or so cars currently, but even so the problem is apparently quite rare.
So: it's hardware-related, and the fix is a new MCU.
Tesla also proactively applied a fix to the C-pillar chrome trim (Loctite), and fixed my off-center steering wheel.
The Rocklin facility doesn't have any HPWCs yet, and wasn't able to charge my car enough for me to make it back to Reno. I took a 15 mile side trip to the Folsom supercharger and grabbed a quick 100 miles of range in about 20 minutes. I wanted to grab some Tesla goodies (a hat, maybe) but they don't have any apparel yet either, sigh.
Overall, a good experience with quick a competent service. Plus, I love the drive through the mountains.