Don't worry about oldquestiontextformat. There were technical reasons why it had to be introduced during the upgrade from 1.9 -> 2.0, but it is not used any more. When you upgrade to 2.3.x, Moodle will get rid of it for you, if it is still there.
You do not need to delete the questions or the quiz settings. Once you have fixed the tables listed above, all your quizzes should start working.
Note that, once everthing has been initialised, you may be able to get this information from the $PAGE global. This has the advantage that it will not use extra DB queries.
This is worrying. Please can you try to work out what is going on.
The hints only show up is 'Feedback' is set to be visible. Can you check that?
If you cannot resolve the mystery, can you export the question as Moodle XML and attach it here?