I just managed to get Nest Protects into HA. The workaround works both before and after migrating my Nest account into a google account:
I’ve setup a seperate Homebridge ( Homebridge ) instance (in my case, the Oznu Docker container in my Synology NAS ( oznu/docker-homebridge: Homebridge Docker. HomeKit support for the impatient using Docker on x86_64, Raspberry Pi (armhf) and ARM64. Includes ffmpeg + libfdk-aac. (github.com) ).
On Homebridge I installed Homebridge nest ( chrisjshull/homebridge-nest: Nest plugin for HomeBridge (github.com) ) .
After this Homebridge detects my Nest Protects, and then I added homebridge to HA, via the HA native homekit controller. (it was detected automatically). I haven’t been able to check CO and Smoke alert, but presence detecting is working nicely.