If charging to 100% and leaving the car idle is really bad, then I suggest Nissan consider that after x number of hours sitting at 100%, and with the driver's permission (i.e.: where a driver has consented that sitting for that numbers at 100% would be inadvertent and they would like the matter addressed automatically) that some of the energy (whatever Nissan deems would be essential to battery health) be released by running the climate system.
Isn't there sort of an analogous feature in how BMW approached their H2 internal combustion engine 700 series vehicles? My understanding from talking to an engineer about 10 years ago is that it had a fuel cell (used for HVAC? or something else?) that would use energy when the pressure in the liquid H2 tank was deemed to be getting difficult.
This would reduce overall energy efficiency numbers on the person's Leaf, but would help preserve the life of the battery. Maybe the parameters could be set so that it was a relatively rare occurrence. Perhaps, for example, if an owner too many times surpassed too many hours sitting at 100%, then instead of just kicking in the battery-preserving energy release feature, the vehicle could start declining to charge to 100% until the owner indicated they understood that both batteries-idled-at-100% and energy-release-wasting-energy are to be avoided, if possible, at which point a counter would be re-set.