No iMovie Main Window All of a sudden, starting up iMovie doesn't give me a main editing window. Most of the menu items are grayed out.

Mojave on a Mac Pro (Early 2008).
You're running an unsupported OS, things are likely to not work as expected. Just because you can squeeze Mojave onto an old Mac doesn't mean you should.
Mojave needs a Metal-capable GPU &amp; iMovie is very probably using Metal.
Even a 5.1 can't run Mojave without one, let alone a 3,1. 

The 3,1 isn't supposed to be able to run anything newer than El Cap, so why not try going back to that?