Here is another shot at trying to make my mod work with M-Preview (Release 2). Note that I have not tested this.
Please flash this zip with TWRP 188.8.131.52 as there may be issues with newer versions of TWRP. I have not had time to look into those issues yet.
If the "advanced" version of this mod (with noise suppression) does not work, please try to install the "simple" version (without noise suppression).
If I remove the noise suppression mod (after flashing the system.img file so that I start from scratch) (and ensure the other settings are the same), it works! It boots up and both mics work!
Unfortunately, it seems the M Preview has issues with bluetooth sound on the N5 (some say audio streaming works, but connecting to another device won't work as well), and even though I don't use it, I'll wait for an update before jumping ship. So - I'm reverting to 5.1.1 - but now, everyone knows that the simple mod (without noise supression) works now on M Preview 2.