It may be possible to get past that, I’ve seen people disassembling the battery to get the BMC and connecting the DC power supply to that instead.

It sounds way more risky than OP’s initial idea. I wouldn’t recommend taking apart batteries.

I don’t think I have enough knowledge to solve this or say anything for certain, but I wonder if the power button is treated as an external keyboard and is getting ignored in tablet mode?

I’m using an app to use my Android phone as a webcam, which works great (good low light performance!) provided I can stand it up somewhere above my laptop screen (finicky). You can accidentally knock it over. Might be worth considering if you don’t use the camera much - otherwise an external one will probably attach better and he more stable