I finally got it to work. Hopefully this will help other people running windows 10 for updating the RS to the latest image.
Before installing any drivers, follow these instructions to disable driver signature enforcement:
Then, install drivers as usual. I installed the Edison drivers via the edison setup and placing IntelEdisonDriverSetup1.2.1.exe in the installs folder (intel workaround is here http://files.emlid.com/firmware-reflashing-tool/IntelGUIWindowsWorkaround.pdf) , and also installed winUSB drivers with zadig (I don’t know if this is necessary or not).
After this I was able to use the Reach firmware flash tool as normal.