I searched pretty thoroughly, guys. I found a bunch (dozens!) of threads about permissions, plenty of which had 0 or 1 reply which didn't help much and nothing which I found gave me enough info to figure this out.

What I did find got me as far as I am now... which is probably 3/4 of the way there.

can you access dashboard if you log in as another registered user?

Yes, but it took a while to figure that out. I had to add "registered users" to the View and Edit permissions while I was in Sitemap as the admin. I showed system pages and then changed those permissions to the Dashboard page.

Since that worked, I figured it would also work if I made sure to change those permissions for the Sitemap pages, but I did that and it didn't work. That particular user can log into the dashboard and click the "Sitemap" button on the top left, but then it brings up a page that says "You must have access to the dashboard sitemap to search pages" where normally you would see the list of pages.

...which is weird because I gave all the Sitemap pages the same permissions as the Dashboard page.

The only permission I didn't give for the Sitemap pages was "Guest" ...but I see no reason why I should allow Guests to edit pages. I've given Edit permissions to Registered Users and Admin for the Sitemap pages.