I think the main problem is that the existing manual grading code expects to only work after each question is 'finished'. And, for essay questions, there is no way for them to get to 'finished' state, until the whole quiz attempt is submitted.
I wonder a bit whether what you are doing is really a quiz, of whether it is better thought of as a new actiity module. (However, if using quiz gives you enough benefits, then no harm it in.)
I think that the core of anything you develop needs to be a new question behaviour, which allows students to edit their response, and teachers to add comments in any order. (Or, would there be a single teacher comment which keeps getting edited?) Could be nice if the teacher can add separate comments, each of which gets a checkbox, so the student can mark it as done when they think they have attended to it? (See what I mean about, perhaps this is a new activtiy type.)
In addition to the question behaviour, you might also want to create a quiz report, like the manual grading report, so the teacher can quickly find all the questions where the student had edited their answer since the last teacher comment, so they can get there and comment again.