A couple of reminders:
An AGM battery typically does not "sulphate"......much.
It also does not lose water from electrolysis; thus no vents.
That is under "normal" conditions.
If severely abused, all bets are off.
As for the battery in question here, you now "have it's attention" and it would be best now to switch to a low capacity tender for a few days and see what happens. While it is not likely, bad things can happen at high charging rates like that if, for instance, a plate shorts out.
I still don't think it is worth messing with. Even if the voltage comes back up, it likely won't be able to supply anywhere NEAR it's rated capacity anymore.