Tim Hunt

Have you read https://docs.moodle.org/39/en/Question_sharer ?

Instead of a local plugin, (ab)using a https://docs.moodle.org/dev/Quiz_access_rules plugin for this might work.

If you want wording changes like that, are you, in fact, looking for the en_us translation of Moodle?

For the other change, I think you would need to do https://docs.moodle.org/dev/Overriding_a_renderer.

If you just try to understand oumultiresponse, you will only be seeing a small fraction of the picture, becuase it is built on the core multiplechoice type. You need to understand a bit more about how the core question type works.

I think you should be able to achieve what you want just by overriding start_attempt and apply_attempt_state. If you do that right, then probably you don't need to change the renderer at all.

And, probably, more important that trying to follow what the code is doing, focus on the data structure. What do the various files of the class mean? https://github.com/moodle/moodle/blob/master/question/type/multichoice/question.php#L60(Hmm. Wouldn't it be nice if the person who created this question type had bothered to write PHPdoc comments. I wonder who should be blamed for that )

Nothing to help with existing quizzes.

However, at Admin -> Plugins -> Activity modules -> Quiz, you can set the default settings used for new quizzes.