Electric Mini Countryman Limp Mode and Gearbox Issues

The symptoms you have described do unfortunately sound indicative of a transfer case issue. Gearbox slippage is never a promising development. Given the mileage of your vehicle, I would say it is imperative to retrieve the fault codes from the relevant control modules as a first step.

How exactly do I retrieve fault codes, and which modules should I be focusing on? Also, is it safe to drive in the meantime, or am I risking more damage?

Regarding retrieving fault codes, that does require diagnostic equipment. Most garages possess the appropriate tools, although some sophisticated OBD-II readers may also suffice. Given the symptoms and the possibility of ABS involvement, focus on the ABS control module, transmission control module, and the motor electronics control module. As for driving, I would strongly advise against operating the vehicle any more than is absolutely necessary. Continuing to drive in limp mode could exacerbate the problem and potentially lead to more extensive, and more costly, damage. It would be most prudent to transport the vehicle to a qualified workshop for thorough diagnosis.

Okay, massive thanks for the help. I took your advice and got it checked out. Turns out it was a glitch in the control module after all! They re-flashed it and everything is working perfectly. No charge!