I dunno if the proper term is plastic but certainly in VAG cars, the leather is coated with some plastic type material so it's not "true" leather.

You can get headlight restoration kits that supposedly work really well on glass. Wetsanding is another option.

I use gliptone leather cleaner and a brush. Dirt tends to ingrained in creases etc.

No idea of the quality of the leather of BMWs but modern leathers tend to have a layer of plastic over the top which means no need for dressing (which would normally sit on top).

Groundnut oil works best and is essentially what you're using from peanut butter. I've seen people on DW use hairdryers to heat up the plastic and turn black, which can then be dressed and tends to stay black for ages.