Yes, metadata can be displayed on both. I know some E38s had rear MIDs and factory nav, that's why that option is multiple choice 🙂
A compatible microphone may work at the factory location, but I imagine it would not work well because it'll be hard for it to pick up your voice. Most aftermarket mics state that they should be installed in the corner of the cluster, which makes sense but is aesthetically unpleasing.
In any case, I'd actually recommend hijacking the OE harness for a non OE mic. That means running two wires up the driver A pillar to the OE mic harness from where ever your mic has been installed. As long as the factory ULF or TCU are not installed, it'll be fine.
All E38 and E39 models that I have come across has the 3 pin white SES connector in the trunk that contains the mic positive and negative signals, so my kit will come with a jumper wire to connect that to the BlueBus.
The PCB mentioned is the OE mic adapter board. It takes the homelink connector and steps down the 12v to provide 5v to the OE microphone. The other side of the board decouples the DC voltage with a capacitor and outputs to the factory harness. You can see more about it here: https://github.com/tedsalmon/BlueBus...r-Installation
Note that the board pictured is a prototype.
Are you using an Intravee for the 5-blink? I've been asked repeatedly for this feature, so I'm actively working on implementing it.
What I can say about the factory mic is that it has way better gain than a bog standard mic. It also, allegedly, has superior noise cancelation abilities, but I don't think this matters much as my BT adapter has algorithms employed for this.