iMovie 9.0.9 is not entirely compatible with operating systems from El Capitan and later, including High Sierra. Lots of
issues have been reported.
You can use iDVD with latest version of iMovie (iMovie 10.1.8) and High Sierra. It just doesn't have the feature of a direct upload to iDVD. What you do is share your movie to your desktop and then drag it into an iDVD open project window. As for using 2 montors, you should be able to do that as well. Where have you heard information to the contrary?
In any event, if you decide to update to current iMovie you still can keep iMovie 9.0.9 on your computer and use it to whatever extent it will function with High Sierra.