Sorry to hear the grief you're having with the window and air bag light. I'm Surprised that it's the passenger seat sensor as your fault came about after removing your door loom whilst battery was still connected.
Did you give the car to a good Indy or dealer ?
I am surprised that members from your area haven't picked up on your thread as yet, there is a serious amount of knowledge held by the senior members on here.
I keep mentioning a good indy as they are generally very helpful and don't have the overheads of main dealers so are more empathetic to your situation.
If you're confident with the garage you have gone to and they are BMW experienced then that's great and hopefully this will be the end of your troubles.