Clean up old iCloud videos without breaking iOS iMovies
The feature I'm looking for exists but was hard for me to find initially.

The Share menu will, for some destinations, allow you to choose between an iMovie Project and a Video File. The iMovie Project option, as a side effect, embeds all referenced video clips.

The ideal for me is to share to iTunes. This is a poorly-labeled choice, but what it actually does is make an archived copy of the project on local iPad storage. That archived copy can be restored later (no use of iTunes necessary).

So, my workflow:


For each iMovie project, Share to iTunes as an iMovie Project
Delete the unwanted older videos from iCloud Photos
If an iMovie is now broken, delete it and restore from the archived copy in the iTunes tab of the Project Import menu


The ideal would be a one-click embed videos command, but as a workaround, this isn't too bad.