So the TFT does not allow the Nav to pair with it and route its instructions through it. I am not at all surprised by that, as to do so would mean the TFT having to be able to simultaneously connect to a mobile phone, a satnav, and a BT headset and route all the incoming/outgoing audio correctly between them.
Can you not instead get your BMW BT headset to pair with both the TFT/phone app combo and the Nav at the same time in the same way that third party headsets like those from Sena can do? The two devices will then just act as multiple separate input sources to the headset.
Could be an issue which one takes priority, but depending on what mode you use to pair it, normally the satnav input will blank the other source for the duration of an instruction, but will then stop transmitting and the other source will resume. The information below is fairly specific to Sena, but might be useful.
Here is the Sena guidance on pairing modes: https://support.sena.com/hc/en-us/articles/217518866-The-Different-Pairing-Modes-on-a-Sena-Headset-
Here is a maybe more useful explanation from a forum: http://www.spyderlovers.com/forums/...ng-Multiple-Devices-to-the-Sena-SMH10-Headset
Cheers for that will have a look