House battery "repair"?

Noise.

Disconnect them from loads - yes. And leave them floating on solar or whatever you have.

Otherwise, - OP saw what could happen.

New AGM - without floating charge - would lose about 10% after 4 months at 40-50F.
Flooded, "near full" after 6 months?... If stored at the North Pole, maybe.

Replace it. Not covered under warranty. A battery that has been frozen can explode while charging.

Is there a way to save this battery? Likely no.. This is one of the problems with MARINE/deep cycle batteries, they are really starting batteries and when they die,,, they die.. Though sometimes you do get lucky.

Try a slow charger, like 10 amps or less for 3 or 4 hours, then use your multi-stage converter to fill it up.

But be aware. It may not last long, you may have lost capacity or it may have a shorteed cell (in which case it will nto charge)

True Deep Cycle batteries can go down to half full safely, and stand a much better chance of recovery if you "OH S***!" them but... Even then in your case I'd not hold my breath.

if the OP had simply installed a battery disconnect switch, his battery would still be good.

my two 12v deep cycle batteries sat for 5 months in storage.
because i installed a $6 disconnect switch, they were still nearly fully charged at the end of the 5 months.

JUST FOR DRILL-----

Charge battery on an old-fashioned wheel type battery charger. Charge on LOW setting until all 6 cells are not only bubbling, but roiling. Stop charging if you run your hand over any part of the battery and it feels hot.

I believe Wall-Fart only allows a one year warranty and the pimply faced Auto Dept. "Associates" are told to be strict about warranties.

Finding a garage that has an old-fashioned charger may be the hard part of this grand plan.

Try a trickle charge