With some effort using ProTools, I now have both seats in my 2013 M3 disconnected and no codes.
It required one non-obvious step. (See below)
1. I first disabled all the mentioned codes which in ProTools are found under Safety->Airbags->Coding->Expert->Equipment.
(The search only works when you are in the correct category.)
Upon saving the changes I of course got the Airbag light.
2. I then shut off and disconnected the seats.
3. Unfortunately I still got the Airbag light and codes 93BF, 93AE, 93AF; the last two are the Active head restraint codes. I double-checked that AKS_LINKS_1_CD and AKS-RECHTS_1_CD were disabled and they were.
4. Next I added RECA to the VO. In ProTools this is under Vehicle->Vehicle Order->Edit->Options. I saved that, restarted, but still got the Airbag light.
When I added RECA and saved it, ProTools noted that this changes factory settings in both Airbags and Footwell module and asked if I wanted to change those to the factory settings. Since I didn't want my Airbag settings changed again I declined.
5.
SOLUTION - I went back to the VO in ProTools, disabled and re-enabled RECA and it again asked if I wanted to change Airbags and Footwell to factory settings. This time I unchecked Airbags, but left Footwell checked and selected "Re-Code". Seconds later the Airbag light turned off and it stayed off after a restart.
So, it appears there is another code in the Footwell module which I (we?) don't know about. At least on a 2013 M3. That might be why the OP had success by adding +RECA.
Sorry, I don't know if ProTools would have correctly disabled the relevant Airbag settings for +RECA if I had let it.
Thanks to everyone; I could not have done it without the info in this thread.