[chriswarbo]

There's no need to convert all TeX back to markdown; the converter only needs to recognise the sub-set of TeX that it produces.

If anyone edits the TeX to fall outside that sub-set, it's reasonable to just embed those parts verbatim in the resulting Markdown.

[j2kun]

Exactly. And I have a very strong feeling that without the ability to collaborate implicitly (at least on the text/math mode part), no replacement for LaTeX will catch on in my community (mathematics).