I have done a terrible mistake and upgraded my macbook pro to macOS High Sierra Developer Beta.

I experience multiple malfunction behaviours, including:

* Missing files right after upgrade.
* Machine does not go to sleep when closing the lid (this might be the cause for other issues, since the macbook got VERY hot in my bag
and shut down forcibly - twice...).
* Mouse and keyboard does not function after login screen (not even USB mouse and keyboard. I had to connect via VNC to control the machine).
* Finder won't launch (`The application “Finder.app” can’t be opened.`).

I would like to downgrade back to macOS Sierra (assuming the problems I described are reversible), but to avoid losing all data. Is it possible?