I would love something like a rich-text editor
but I do want that basic visual of formatted text without having to mentally parse the markdown code, so I'm not looking for a two-phase solution with VIM and LaTeX, a two-pane markdown editor with live preview, or a note-taking app.
I'm not sure to understand your requirements in regards to Markdown 'behind the scene'? I mean, if you don't need visual preview you should be able to use any text editor or word processor and save your work as a text file, right?
But I would say: Any recent version of LibreOffice, on a computer without Internet. It's WYSIWIG and even though it's not 'markdown compatible' one can easily write using Markdown with it.
Before switching back to pen and paper for drafting all texts (no more Markdown needed at all ;), that's what I used to use.
No emulation, no hacks, no nothing to install. It comes with a decent speel checker (at least Grammalecte, in French and it used to work with Druide Antidote, non-free English and French speelchecker but they stopped offering a Linux version). Also, no two phases editors. Just plain WYSIWYG. And with styles it should even be possible to have kinda formatted/WYSISWYG Markdown combo but I never really tried to devise it as it was not worth the hassle for me: even on long form writings I barely need more than the occasional italics I can easily type by hand in Markdown, with the ability to save the work in a text file, and also LO can export to HTML or epub (using the right extension, as the native export is meh).
BTW, Jonathan Franzen used to install a fresh copy for MS Word on a cheap laptop without WiFi card and whose Ethernet port he filled with glue so there was no way for him to connect it to the Internet. But that was before Microsoft switched to a sub-model that required a monthly check online to let the user run Word. Don't know if he still does that, and how.
I used an old ThinkPad (X220) that was more than powerful enough to smoothly run LO (edit: with whatever was the latest release of Debian), hooked to an external display because its own display was shitty at best.