A few days ago, I showed someone how to modify the website I built for them. I gave them only "content edit" mode.
The one thing I noticed was the slots, while still called out as a different color - it would be helpful for the content editor users to know what those colors mean. Could a message be attached to the slot that tells them it will change across the entire site? (see my screenshot to see what I mean).
Side note: She loved the content edit mode as it was super easy for her to understand. She previously had a WordPress website and hated it.