I don't see the containment kludge being an issue. the outlet of the container could have a 2-way valve,,, venting out if internal pressure raises above standard and normalising (venting inward)when the aircraft drops below cabin-altitude oressure. the vent could discharge into a simple baffle-trap (think sink-waste) and final discharge could be arranged tat the tip of the tail-cone.
the sales-goons could promote this feature as "jet augmentation" to get you to safety quicker,if the battery failed
any fluids not caught by the trap would discharge into the slipstream clear of structure. (maybe a hazard on the ground, but you can't have everything on the wish-list when bodging!
As these "standby" batteries are clearly NOT just that, the 2 controllers could "talk " to each other....should ground-staff over-discharge* either battery, the other could automatically start the APU which would then recharge BOTH to meet despatch requirements.....
*(over-discharge means reduced reserve BUT STILL ABOVE ON-BOARD RECHARGING-LEVEL.
back to battery construction......the coiled,"swiss-roll" method would still suffer similar stresses to folded construction...plates swelling would pressurise the seperator-membrane, shrinkage leave voids. linear expansion would "shuffle" them ,one agains another "wiping" the paste into uneven clumps....not saying this doesn't happen with present folded-construction, but simply restating my opinion that wound circular cells have weaknesses as well.
many posts back, i suggested,- each subcell in a thermal/electrically insulating pot tails of fusible material connecting to an internal busbar to main-cell stud.
thus, any individual subcell going T.U. would self-isolate allowing the cell to carry on delivering full voltage at 2/3 capacity.
An aditional safeguard would be to make the inter-cell straps as fusible-links....the problem there is that current sufficiently high to melt them would mean the battery was already unserviceable , so this would be s "stable-door" action.
Every sub-cell should be monitored...24 delicate devices linked together with the only control being on batches of 3 ,doesn't and hasn't worked.
Any of the millions of model-aircraft and Heli. hobbyists, who ,as a matter of course, run Lipo's to the edge, could have told the boeing bean-counters and engineers that their design was fatally flawed. (not to mention the cellphone and laptop manufacturers who'se products are used by totally untrained and non-technical consumers.....other than ONE recall, BILLIONS of active-service hours...and they don't charge 16K for a laptop-battery, charger and control-system!
IMHO, this is another short-term kludge to get the revenue rolling again before the liability takes them down.