Once imported, is the menu in the menu viewer?
No. There are no menus in the viewer.
Can you import a menu from the library?
I don’t think so. Which library?
You say you reset the library preferences; do you mean the Encore preferences?
I mean the .plist file.
Was 5.1 working okay previously?
Yes, the last time I used it, a few months ago, I think before the upgrade to High Sierra.