ISSUES RESOLVED (for me): after watching my JS error console, I did the 2 steps below on my remaining 5 "problems" installations. This fixed all my remaining problems, including (a) our Edit Post sidebar issues, and (b) an Appearance/Widget frozen/inop widget icon issue I didn't know I had.
These steps are only what worked for me. I have no obvious explanation why all my installations weren't affected or whether they'll work for everyone, but:
1) in my affected wp-admin folders I re-uploaded these 3 files: load-scripts.php, load-styles.php, widgets.php
2) testing the affected control pages, I found I had to refresh the page in Firefox to see the fixes take effect, but work they did.
To re-test, I logged off all installations, closed Firefox, and started it again. Logging in, the fixed functionality remained "fixed" - without refreshes or other gimmickry.