I realize this is ancient knowledge, but I was reminded that problems can lead one to find unexpected solutions.
Per this post, I finally realized I had to cancel my Ulysses subscription. This meant that all of my writing, except formal formatted documents (Pages) and a book project (Scrivener), would move to iA Writer. In some ways, this is a relief as my writing app angst has been removed.
One of the hurdles I’ve faced, regardless of the markdown editor used, was exporting an OmniOutlier outline to markdown (I wish the OmniGroup would add markdown exporting to OmniOutliner). I don’t want to use Pandoc.
Suddenly, I wondered, “What if I used markdown syntax in OmniOutliner? Would it export with the headings for a markdown editor? If so, what export format do I use? iA Writer will not open OPML files. Plain text files do not format correctly in iA Writer when exported.” 
Then, I went further and decided to challenge an assumption. “What if I export from OO to DOCX (indented) and try to open it in iA Writer?” I assumed that I could not open that doc in iA Writer. I was wrong, gladly so!
Here is how this works:
- Create an outline in OmniOutliner using markdown syntax (at least headings, I did not try other syntax but I’m guessing it will work).
- Export the outline as an indented Word document.
- Open the Word document in iA Writer. You get this (Note: I added the code block for the TOC from within iA Writer): .
I think that is pretty cool. 
Now I can create extensive outlines in OmniOutliner, export them to iA Writer and then write.
You advanced users and programmers probably think I’m crazy, that this is “101” stuff. That is okay, I probably am crazy! 
But, I’m rather delighted at this discovery. I can now use iA Writer enthusiastically with little friction and I achieve my app subscription free goal.
Not too shabby for Monday. 