If you want a markdown editor that shows the actual markup, I think you’ll want to stick to CodeMirror (ProseMirror is more of a WYSIWYG-style editor). CodeMirror 6 allows native spell-checking, and it seems that it should be possible to add spellcheck=false attributes to individual lines to disable spell-checking within parts of the editor.
There’s no proper Markdown mode for CM6 yet, though, so you may want to hold off a bit.