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A dying 12v battery can also wreak all sorts of havoc on how the car runs. Definitely definitely replace it. One morning I came out and found my car would start via the IMA system but the dashboard was completely dark and the car would not exceed 19mph, and ran really rough. I was terrified something truly bad had happened, but replacing the 12v battery fixed it.
Based on what you've said, I think there's a strong chance the issues you're seeing with the IMA are 12v related, either the ground straps, battery or both. Get those replaced ASAP or it might get worse and you'll end up stranded somewhere. All existing hybrids all l still need a functional and healthy 12v system.
The reason to unplug the BCM with a bypass is that with a really bad IMA, you won't get 12v charging at all and the car can actually die while you're driving it. By disconnecting the BCM the car will run and drive fine (assuming your 12v battery is ok too) you'll just get a temporary loss of 12v charging at high RPM and your headlights will dim a bit at low RPM. Reason being, the voltage coming from the IMA motor varies greatly based on RPM, anywhere from asking 70v at idle to 300v+ at redline. The DC-DC converter can run with an input as low as 70v but not at full output, and when the battery is installed it's always fed around 165v. At high RPM the DC-DC temporarily disconnects because it's input voltage is too high.
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