Just did head gasket, now won't start

2012 Prius V, 200k miles, just DIYed the head gasket replacement and intake manifold and EGR cleaning with help from everyone here. EGR cooler was completely blocked, so were passages to cylinders 1 and 2. So much oily carbon in the intake. Everything is assembled and i wanted to enter maintenance mode to get air out of the cooling system, but I'm at an impasse. Turns out it won't start at all.
The whole thing did take about 4 weeks, between getting the head back from the machine shop and doing this a few hours at a time, so the HV and 12V batteries were disconnected the whole time. According to Dr. Prius, the HV battery is at 53% state of charge. There were code in Dr. Prius and OBDLink, I'll try again tomorrow and share screenshots.
12V battery was around 11.6, and i wrongly assumed it would charge leaving the car in what I thought was ready mode, but it drained itself.
Connected to our pickup's battery with the truck idling, got the 12V up to 12.7. Trying to start still just shuts down the car. Start button is orange, then with foot on the brake it's green, but when you press it, the dash goes dark except for the mileage and a flashing car with lock at the bottom right. 12V systems all seem to work. Fob battery was 2.88V, so replaced it with new, but still no better.
At first I thought I was doing a bad job of the maintenance mode sequence, but there's something else. Park seems to work, neutral seems to work, the car will roll. Turning the crank by hand (ratchet) spins freely. 12V systems seem to work, but there's no starting the car.
I did find an aftermarket remote start module wired into the car, but I'm hesitant to remove it since before I took everything apart, the car did run. We did not know this module was even in the car.

Current plan is to disconnect the 12V battery for 5 minutes, reconnect, and check codes. Any other ideas?