That's a great idea. Many forums (looking at you Reddit) only show a small "edited" tag on top of the comment, but they don't show the history of it, also users have to write why they edited the comment manually only so they don't lose credibility when it comes to serious argumentations.

About the markdown editor, I would suggest copying the way MD editors such as QOwnNotes, ghostwrite, Standard Notes, Iotas show the text in the main editor, the MD syntax takes a second place so you can see graphically what you're doing and the result without having to compare between two windows or two tabs