My experience with Mini is that the SE model is so rare they are quite clueless about it - at least in my part of the world. The experience of others here may very well differ.
For instance, you may have seen me post elsewhere on these boards that I was getting a repeating “Charging Rate Reduced” message whenever I charged at home. They couldn’t replicated at the dealer and so insisted it was my house electrical system. I’ll save you the gruesome details of the year long dispute back and forth. They were, it turns out completely and totally incorrect. Thanks to another member here having a similar problem, they finally did a deep enough dive into the battery management system and – wonder of wonders – there was indeed a fault requiring its replacement, at a cost of about $4000. If I hadn’t kept hammering away at them, my warranty would’ve expired and I’d be on the hook for that repair.
Anyway, all that to say, stay the course and stick to your guns. At one point, I had the service manager for the BMW dealer next-door, who also covers the Mini operation, a couple of techs, and a supervisor all standing around the car while I educated them on the difference between domestic and commercial power grid voltages and reason they couldn’t replicate the problem. And me having just learned about it myself from a board member. None of them had a clue. But to be fair to them they did agree to keep looking even though I could tell they thought I was nuts. Why the car didn’t record an error in the battery management system software that was easier to find is beyond me. Apparently they have now flagged this issue worldwide in their tech systems so if anyone else brings an SE in anywhere they are sold with this issue, it’s recorded that at least one poor slob had the problem and the solution is such-and-such.
Apologies - looks indeed like you got most of the gruesome details after all…Perhaps your 12volt issue will be solved in this way too.