Well, not completely though.
You forgot to revert the hook-change in plugin.json:
The "error throwing" part of the com-message refers to
At that point I get ratio === undefined, yet width is also not given. Whereever this $el comes from there, it doesn't have .dataset.ratio, nor .dataset.width anymore.
Can't explain why it's not showing in your ACP list. Didn't have any problems with that.
If it were working, would you prefer that Markdown hook over the post-parse one? Regarding downward compatibility, I'm hoping that there's a way to add hook handlers in an init function, isn't there? So you could check for Markdown and from there on set the appropriate hooks.