Yes, it is possible.
Yes, it is possible.
Yes, you can revert to Windows 8.1 after upgrading from Windows 10 by using the recovery options or restoring from a previous installation.
I switched from Windows 8.1 to 8 by mistake on my laptop, then moved back to 8.1 from 8, and finally from 8.1 to 10 again. That’s possible.