Apologies for replying to myself…
The failed swelling battery is now stored as safely as I can, until I locate and deliver it to a disposal(? recycling?) point which “knows” how to handle failed outgassing swelling batteries. It has had it contacts taped over (non-conductive tape, as far as I know), loosely wrapped in paper, buried inside DRY sand inside a steel bucket, and placed in the coolest available dry spot away from (all) the exits. I’ve checked my (industrial-grade)† CO₂ fire extinguisher (it’s overdue for an inspection 
), which is carefully positioned to be easily but not-riskily accessible in the event of the worse. And the smoke alarm passes its tests.
Whilst I am in the EU where (I believe) shops which sell such equipment(? batteries?) are required(?) to accept them for recycling(? disposal?), the local collection points I know of do not impress me when it comes to dangerous failing batteries. And I’m not very keen on transporting it long distances, nor by public transport…
I am aware the Risks are relatively small — PROVIDED the battery is kept dry, cool, and (as much as possible) in a fire- / explosion-resistant environment.
† My fire extinguisher is one of those models you find in factories, etc. I acquired it by legal albeit unusual means. I am trained in how to use it.
). I also ordered one of those (and a few other minor accessories).