Hi Leonard, and welcome to the forum.
Two things come to mind with your 12v battery problem -
1. A shorted cell within the battery, the result being the dc-dc converter pumping loads of current into the other cells trying to bring them up. Result is heat generated by the good cells as well as the shorted cell, boiling off the battery fluid.
2. The output regulator of the dc-dc converter fails to do its job and a higher voltage is being fed directly to the battery, cooking all the cells. I haven't looked at the circuit so that's speculation.
I suspect the first.
Do NOT turn on the car. That battery is toast and I would simply take it out. Measure that battery voltage with everything disconnected: if the voltage is around 11.4v then the battery has a shorted cell (unless the other cells have now been damaged). Now, attach any usable 12v lead-acid battery to those car terminals (you can use jumper cables to a battery sitting on the ground, being VERY careful to insulate all exposed wiring so as not to short anything), put a voltmeter across those terminals and turn on the car. Hopefully it goes into READY, and that battery voltage should be somewhere around 14.4v when energized. If the voltage is way higher (like well over 15vdc), that would indicate a problem with the dc-dc converter.
Hopefully, all you need to do is simply replace the 12v battery and all will be well.
EVs: 2 Wht/Blu SE Prem., '13 Tesla MS85, 3 156v CorbinSparrows (2 Li-ion), 24v EcoScoot(LiFePO4)
EV Conv: 156v '86 Ram PU, 144v '65 Saab 96
Hybrids: 48v1kW bike
ICE: '88 Isuzu Trooper. Mothballed: '67 Saab (orig.owner), '76 MBZ L206D RHD RV