You didn't mention exactly how iMovie is failing to respond properly to your editing.
However, try deleting/resetting preferences and see if that cures it. To delete/reset preferences open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Possibly you may need to reboot to set the changes. Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.
To revert to your earlier OS you will need to have made a previous Time Machine backup. Go to the Finder help menu and type in "Revert" and select Revert to previous OS from the drop down menu.
-- Rich