Q: Following the instructions here (1-4), and then re-starting the computer, means that I now can share my document via Notes or Messages, but still not via email. What should I do?

Try this.

If Share options or Markup is missing after upgrading to OS X Yosemite or El Capitan

I have followed the instructions (1-4) and that corrects all.   But when I do a restart it takes everything back to the way it was.   No email option in Safari, Preview nor is Mail listed in Extensions.  

Any other fixes?

I have the same issue. It's getting a bit frustrating to have to run the command in Terminal each time I restart the computer.

Same prop, but the return doesn't seem to be tied to RESTART. I restart only a couple times a week.  After running the 'terminal solution' with good results, when I return to the computer, following a period of SLEEP, the problem returns to all the SHARE menus.  (specifically, E-mail is missing as an option in Photos/Preview, etc.)  E-mail is not listed in the Sys Prefs extension options as well.

My problem still exits.   I have had mine run for 3 days and go into SLEEP come back out and no problem.   Do one restart and the E-mail is the only SHARE     menu that then disappears with Safari, Preview, Photos etc....