Tim Hunt

Yes, that is bad data that should not exist. It has come up in the context of upgradeing to Moodle 5.0 (or 4.6). I was on holiday last week, but I think there was some talk about developing an automated fix. See MDL-84305.

In the quiz, there is only the option to do a settings 'Override' for a particular user. Look for the 'Overrides' option in the 'More ...' menu. (e.g. https://qa.moodledemo.net/mod/quiz/overrides.php?cmid=1171&mode=user on the demo site.)

Well, one approach is to turn off Moodle's automatic display of correct answers, and instead write 'General feedback' for each question, in which you give the right answer, and any explanation to help the student understand what they were supposed to learn from the question.

But, you are right, for this question type, with this sort of question, the automatic display of the right answer could be better. You could add an 'Improvement' request in the Tracker. That would be the official way to suggest that is done one day.

What does this do to git history/blame?

Calling the service with page -1 should fetch all the questions.

Relevant code is here, and looks fine:

https://github.com/moodle/moodle/blob/5670447ece698da18e20d0f1984965535487feda/mod/quiz/classes/external.php#L1488
https://github.com/moodle/moodle/blob/5670447ece698da18e20d0f1984965535487feda/mod/quiz/classes/external.php#L990
https://github.com/moodle/moodle/blob/5670447ece698da18e20d0f1984965535487feda/mod/quiz/classes/quiz_attempt.php#L834