Safari (and system) share menu - Messages and other items missing in Sierra System configuration


Running Sierra (10.12.5)
MacBook Pro 13" (Early 2015) 


Issue

Messages and many other options are missing from the system Share menu, both in Safari and in the Share Menu extensions pane.



These among others are missing:


Messages (most annoying)
Facebook
Twitter
Linkedin
Vimeo
Flickr


Here's what I've tried so far with no success:


Update to latest Sierra 10.12.5
Run this command line to force-reload launch services
Disable and re-enable iCloud contacts
Disable and reenable System Integrity Protection


Related observations


Under Preferences -> Internet Accounts, I cannot check the box next to 'Share Menu' for Facebook, Twitter, Vimeo etc, even though accounts are correctly configured.



Really appreciate any help!

Update 7/11:

I just got an old 2011 MackBook Pro repaired, and upgraded it from El Capitan to Sierra. The Share menu works as expected, with Messages as an option. Still not working on the machine with the problem.
https://support.apple.com/en-us/HT203129
This is the fix according to apple. I have a very similar problem but this fix enabled only one or two extensions for me. I ran it 3 times. I'm still looking for more info