Water under 2nd row rear seats caused main electrical failure and JLR trying to wriggle out of warranty and charge me £12k for repairs. Car 2y old (Sept 23 D5). I'm going to get an independent assessor in, but what are the common issues here?

Currently have courtesy car below as they also found a coolant issue, but i'm about to become carless and on my own with 2 small children.