Sorry about not giving an update earlier, but life's been pretty crazy for the past few weeks. My apologies another long post, but going into detail seems to help with troubleshooting and also may help others with the same problems.
The day after my previous post (July 6th), I brought the car in to the dealer and talked briefly to the same service rep I had worked with before about the bulletin 14114 and the sensor/wiring harness/etc., and he seemed to trust that knew what I was talking about. Fortunately, they offered to only charge me for parts and no labor since they didn't fix it the first time; it was done the next day, and they replaced the sensor and wiring harness (total about $60).
I also ordered one of WOT's sensor plugs, but I'm guessing I'm in the backlog queue from his vacation, and it hasn't come in yet.
The system basically worked fine until yesterday, except for a probably unrelated separate fault. Near the beginning of August, I had gotten the not-as-dreaded "AC off due to high engine temperature" fault on a long drive a little after going into charge sustaining mode, but it was cleared with a Scan Gauge at the end of that drive; this showed the not-helpful B2AAA code. On the return trip, the check engine light came on with no vehicle messages right at the end of the drive IIRC and was also cleared. A few days later, I again got the check engine light with no vehicle messages; my Navdy HUD (I guess I'm an aviation geek) had just gotten a firmware update to display OBD codes, and it showed P2181 before I cleared it with the Scan Gauge. I assessed that the the 2nd and 3rd occurrences were probably transients that tripped the BIT but would have been cleared on their own if the car was restarted 5-10 times without the out-of-tolerance being detected.
Yesterday, it was again just around 100 degrees F in LA area, and on my way to meet with some family I got the "service high voltage charging system" fault again; the HUD showed P1E00, P1FFD, and P1FFF. Assuming the problem had come back yet again, I unsuccessfully tried to divert enroute to a couple area dealers to see if I could pay someone to do a quick reflash of the HPCM2 module without further troubleshooting; as it was Saturday afternoon, one place was already closed, and the other had no techs with that skill set that late in the day. I went to do research and saw the stuff in the main WOT sensor plug thread with the 24.7K ohm resistor (on about page 31-32), so I stopped at Fry's to get some resistors before continuing on. While at the parents', I soldered the resistors (22K + 2.7K) and tried to test fit them in the sensor cable a couple times, as I decided since the sensor had already tripped the HPCM2 I wouldn't make the situation any worse by pulling the cable.
When I finally got home, I tried to check the codes with the Scan Gauge just for the heck of it (and to clear any spurious codes that might make the service techs do other unrelated procedures unnecessarily if I were to bring the car in Monday). The HUD had said the 3 codes, but I noticed the Scan Gauge showed 4 codes, with the additional one being P2181 again; no additional vehicle messages other than "service high voltage charging system". I tried to clear the codes at that point, and miraculously the check engine light went out, and the vehicle messages page went from 1 to 0 vehicle messages. I put the car on the charger shortly after, and it's currently charging normally so far.
Not sure why I was able to clear it this time when previously trying to clear codes has been fruitless.
I'm considering trying to install the resistors temporarily until the WOT plug arrives, but I'll make a post about that in the main WOT plug thread, as there seems to be some recent discussion there.
Thanks again, everyone, for your feedback and helpful inputs.