P0A4B workup... >1V on HV control ECU M5 connector

Here's the background (may not all be relevant)

1. When out of state my 2009 Gen 2 Prius with 235k miles threw up the red triangle of doom. I took it to a shop, and they reported "TRIANGLE OF DOOM...p1121 water valve code...p0a4b transmission connectors...remove and replace water valve and bleed coolant in inverter"
2. The next day the check engine light came on, and a couple days later the red triangle came back. I was at home so I was able to scan with techstream, and got P0A4B, P0420, C1242, C1310.

Of these codes, P0420 (Catalytic converter) seemed independent, C1310 (Malfunction in HV system) seemed secondary, and C1242 (Open in IG1/IG2 power source circuit) seemed possibly secondary , leaving P0A4B (Generator Position Sensor Circuit) as my likely culprit. (the P0420 and C1242 were also intermittent when I restarted the car).

3. After resetting the codes, none came back (only driven briefly a couple times though).

So I am focusing on diagnosing the P0A4B, using the workup from this thread: https://share.qclt.com/%E4%B8%B0%E7%94%B0%E6%99%AE%E7%91%9E%E6%96%AF%E5%8E%9F%E5%8E%82%E8%8B%B1%E6%96%87%E6%89%8B%E5%86%8Cpdf%E6%A0%BC%E5%BC%8F/repair%20manual/04pruisr/05/21bpm/0a4b253.pdf

So basically I am checking the connection from the HV control ECU to the MG1 Generator Resolver port. Following their directions

1. I check the pins in port on the ECU, measuring voltage to ground, with the car turned on. All 6 measured pins were >1 v, either 2 or 4 something volts, except the CRFG pin.
2. I don't have long enough cables to check the resistance of the connector from one end to the other, so I didn't check that
3. I checked for shorts from the connector to ground as specified, and did not find any.
4. I checked the resistances of the Generator Resolver port as specified, and everything was in spec.
5. Just for fun I checked for shorts between the different pins on the connector, and found nothing.

I also checked for corrosion or damage to the cables or ports but everything looked very nice and clean. I cleaned both sides of the connection on the generator resolver with brake cleaner just for good luck.

Any help interpreting the voltages on the ECU pins? According to the workup, if the voltages are above 1v, you are just supposed to "REPAIR OR REPLACE HARNESS OR CONNECTOR". But this seems odd to me... it's not like they have crazy voltages, and the car isn't even giving a code any more... Does it really mean the whole ECU is cooked?

 

The red triangle both times began at startup. The check engine light though started after driving for a bit (and also disappeared while driving).

The 12v battery had previously died a few months ago in the Vermont winter, and I had to trickle charge it back to life, but it hasn't had any issues since then.

That does seem plausible as contributing, but I still can't make sense of the diagnosis of >1V pins on the ECU, and I'm not sure how the battery could be causing that symptom, while at the same time throwing no codes and generating no other symptoms.