2015 Land Rover LR4 - Replace Battery, Reinitialize Battery Monitoring System & Other Battery Stuff

The "Other Battery Stuff" is probably more useful, and certainly more interesting, than the actual bit about actually changing the battery.
Error Messages
OEM Battery Configuration
Can we install a Larger Battery (an H-8) ?
That Mystery Pigtail
Quiescent Drain Current (and vehicle shutdown times)
AGM Batteries and Battery Monitoring Systems
Pyrotechnic Coupling