After enabling the "Edit mode", the Single view feedback display fields shrink to just very small input areas, where editing is very cumbersome.

I believe there should be a pop-up option that would load the entire contents into the HTML editor's mode where the entire text of the feedback could be handily (post)edited, and other Grade details controlled.

The "quick editing" Override checkbox toggle functionality is insufficient.

Cf. the modal popups to "View feedback" and especially "Edit the grade" available in the Grader report.
I think the Single views should be just as powerful.