That great news!
If the mic signal is passive, I doubt the B&M will throw any errors if you fully disconnect the mic.
e,g, unplug it from the B&M or take the pins out of the connector. That way you don't have to "share" the signal. If you keep it plugged in to the B&M you might get into a whole lot of trouble and noise due to ground loops.
I doubt there are any useful B&M voice commands if you are going to use the new HU for media, navigation and voice calls.
I'm trying if possible to not actually remove any pins or cut any wires so if others want to do this modification but don't want to ruin their wiring loom (say they wanted to convert back to OEM for resale) they can.
But I do appreciate your concern.
It does all depend on the power and ground filtering in the HU. I'd suspect 0V planes used for audio would be isolated using chokes but that's going too deep into EMC design.
It's one of those situations where it's quicker to try something and see if there are any issues then adjust, rather than trying to figure out the best situation to begin with.