There is a list of responses, there is a delete all responses, but there appears to be no way to delete one response for one user; or delete the responses a group made whilst leaving the rest. Am I missing something?

Hi Tim,
1.- You wrote: "Oh right! I honestly didn't see that there." A mere "Thank you for your help" would have been more in order.😉
2.- "Related question: How do you delete/complete a response that has been abandoned? (e.g. never completed)?"
Do you mean you (in teacher role) should be able to complete a student's un-completed response? That would be un-acceptable interference, IMHO.
3.- As regards Scorm I've never felt the need to use it on my Moodle sites, so I have no opinion of it.
4.- "Having button "perform action" functionality but appearing as a
navigation tab "change the view" user experience is a pretty strange
decision."
I see your point. Having spent many (too many?) hours maintaining and "improving" the Questionnaire plugin in the past I take full responsibility for that pretty strange decision. You are welcome to create a pull request to improve things on https://github.com/PoetOS/moodle-mod_questionnaire/pulls
5.- "Moodle already has a bunch of user and group filter screens that work fine for managing attempts, ordering, groups, filtering etc"
Agreed, again, if you can fix this and create a pull request, it might be considered by the current Questionnaire maintainer, Mike Churchward.
Thanks for your suggestions.