code p3102 and intermittent triangle of DOOM

hey gang. I've been combing through old threads and decided to finally just post my own issue.
I've got a 2008 prius that's bounced back and forth from the shop for a week now. Initial issue was it refused to go into drive or reverse and had the '!' triangle.
The car will start with all its warning lights on, parking button light blinking, and won't shift into drive or reverse.
The most consistent trouble code my bluetooth obdII checker reads is p3102 but it occasionally throws other ones. It threw P1116 the other day then hasn't thrown it since. The only thing I can think of that was different was today it's hot (I live in Florida) and two days ago when it threw p1116 it was raining and cooler.
I've had it in the shop and they tried a few things: replaced 6 battery cells, then, when it started doing what it is now, I was able to drive it back and they replaced the ECU and the compromised battery wiring harness assembly. I was able to drive it back, like I said, because the issue going into drive/reverse is intermittent. Occasionally no lights come on, sometimes just the triangle, but lately all day today it's been the whole gamut, triangle, all warning lights, no ability to drive or reverse.
Car ran and drive fine for a few days then hit the issue that it's been having after its latest stint in the shop. I've only dropped 500 into it so far plus a tow so I'm not too hurt about it but I really like the car and would love to get it running again. Reluctant to send it back to the shop as I doubt they'll find the issue since it's intermittent.

From what I've gathered p3102 could be a few things, one of which is the AC? I'm using an OBDII and Dr. Prius to check the battery so to my untrained eye all the green bars going up and down look good when i'm on the battery health screen. Is it possible the battery itself should be in a working state but some heat sensor is causing an issue? The 12v battery comes up a lot too, how do I check that? The shop didn't mention looking at it which surprised me. Anyways, let me know. Thanks!

 

couple more things: just went out again and managed to start it. The triangle was gone for a minute then came back. No warning lights other than that. OBDII didn't show any trouble codes at all...that's weird. Only things that were different were: there was a cloud covering the car for a bit and I started the car with the OBDII plugged in. Could that possibly affect the car?