Update:

1 LTI Questionnaire per Provider course and allowing Authenticated users to complete and submit the questionnaire did not resolve the multiple non-respondents issue: Users in other LTI activities (from different Provider courses) in the Consumer course, are listed as Potential users in the activity - and multiple emails are sent. (In my test there are only 5 Enrolled users, but an additional 16 Potential users (the same 5 people) - so 21 emails will be sent (distributed among the 5 email addresses).

So my questions are:

Is it possible to sync roles prior to the user accessing the activity?

Is it possible to remove "Potential users" from having permissions in the activity (there is only 1 LTI activity in the Provider course) - and what is the purpose of having this feature at all?

My last idea is to investigate whether the Questionnaire module could be hacked to identify non-respondents via unique email address and not username...

But perhaps LTI is not the solution here... perhaps I should investigate MNET