You REALLY DON'T want to mess with working on the traction battery - leave that to someone experienced with them. As for the 12 volt battery, I think you misunderstood: the Leaf just isn't good at charging the 12 volt accessory battery, and a chronically undercharged 12 volt battery can cause weird issues with the car. I don't think that that is the case here, but just in case, check the rest voltage (car off for 20+ minutes, hood left open so you can leave the doors shut) of the 12 volt battery with a multimeter. If it's at least 12.5 volts then that likely isn't an issue.
Brilliant Silver 2021 Leaf SV40 W/ Pro Pilot & Protection
2009 Vectrix VX-1 W/18 Leaf modules, & 2 lithium E-bicycles.
BAFX OBDII Dongle
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.