Unfortunately you are going to have to get a dealer diagnosis in order to get to the bottom of this. The scan tools used by independant mechanics (as you are experiencing) are not sophisticated enough.
This could very well be the seat sensor again, but it could be another component in the passenger restraint system that has failed.
$119 is a fair price for a dealer diagnosis. Once the failed part has been identified, you will have to pay BMW msrp for the replacement, as well as the dealer's labor rate to have it installed. This is unfortunately part of the cost of owning a BMW.
Please let me know if you have further questions, I am happy to answer follow ups. Please press accept if satisfied so that I am compensated for my time, effort and expertise. I am not compensated until you press accept.