Okay, seat has been replaced. So here's the feedback.
My driver seat heater had two hot spots, one burnt through the leather quite bad.
Took it apart and inspected the element, yes I tore into it and it is a lot more complicated than I expected, and nothing like others videos on YouTube.

BMW seat is full of conductive carbon fibres that create the heat. They are incredibly this fibres, easily pliable, and tough to break. They are very good quality. These fibres are spread all over to produce even heating across the whole seat. I could not find a kink that would cause the hot spots, besides the fibres are very flexible and no way they would kink and stay kinked.

The replacement driver side seat cushions on the market are in heavily used state. So I bought a passenger side cushion. The leather is in a very good state. The passenger seat has an occupancy sensor and plug, otherwise it fit perfectly on the driver side. Installed it and simply tucked the occupancy sensor plug out of the way and all is good. Seat warmer is functional and leather is good.

Won't that mean lack of passenger airbag function - potentially resulting in serious injury to a passenger?

AM.

I believe he is saying he purchased a used passenger side cushion from a different vehicle and installed it on the drivers seat. That restored his seat heating and the only issue is he had to abandon the occupancy sensor that was already installed on the cushion as there is no where to plug it in on the drivers side. He made no changes to the passenger seat currently installed in his X5

Ahhhhh... must. work. on. reading. comprehension!

AM.