Also, I was going to post tomorrow, but in light of the question here, I'll make it today. My test run is going very well. Grades are being updated but only grades that have changed are being sent. Students who had been forwarding me the rogue notifications have been silent for the last 5 days.
If others want to implement the same change, it involves adding some lines to one file. (/opt/webwork/webwork2/lib/WeBWorK/Authen/LTIAdvanced/SubmitGrade.pm) The proposed fix is provided as an attachment to the relevant bug report.
(There is one extra modification to that file in that I have commented out a debug line that was giving me an error for students that had dropped the course and has not yet been resolved.)
I hope this helps.
- Brian Walton