Well, I left the engine bay open for 3 hours to disperse the hydrogen, and nothing's been connected since. And, to be fair, it was the charger that claimed to be fit and forget, not the batteries.
The grenade reference is a bit terrifying, insn't it? Battery was warm mercifully, not untouchable, so I hope that means some tolerance left. Not that it matters, I'll be binning them soon. I have a part delivery from Tayna now, just waiting for battery 3 which, apparently, DHL lost in transit. Just hope all the cables swap smoothly over from old to new.
BTW, vaseline or not on the terminals?