You can flash the full factory image of 7.1.1, that should work and take you back.
I have tried flashing the NMF26U and the NOF26W versions - phone is not enrolled in beta any longer
rebooted or shut down the phone several times - enrolled-unenrolled from beta program several times
when originally enrolled in beta - side loaded the image - didn't come OTA
When 7.1.1.came out I sideloaded the "V" version - there was an article stating the V was for Verizon, which was later corrected to state it was for Europe. I don't know how, but I suspect this somehow caused the issues I am having now - MAYBE
When I enrolled the Blue Pixel, on the "U" version of 7.1.1, the beta version appeared right away, but I didn't down load it.
The pic above showing adb "total xfer: 1.oox" - after image sideloaded - OR - I it says couldn't flash because version on phone is newer than what I'm trying to flash
I have no idea what to try now - I may just leave it alone and see if the "unenroll" image ever shows up.
This is one area I like computers better - I could format the drive and restore a good image, or throw in a new SSD if needed.
Maybe I could sell the phone as is:
FOR SALE: - Unique Black 32 GB Pixel - used by Grandad to text Grandkids once in a while - no marks or scratches - sits in cupboard 99.67% of the time - locked into the 7.1.2 beta OS -
I've taken up enough space here - and I need coffee - LOTS of coffee.

