I am happy with Multimarkdown Composer (version 3, non sandboxed; you can download it aften buying the app in the App Store). It ticks all your boxes (it just uses separate windows instead of tabs). You can give a try to the demo.
I have used it for hours every day for about one year and is stable (no problems in the last 6 months), functional and a pleasure to my eyes. I use it often together with Marked 2 when I need more control on the export functions or wish to use a grammar checker.
Since you use Editorial you will probably miss RegEx, but you can always open your file with fx Brackets, do search/replace with RegEx and open the file again with MMC.
[I have no connection with MMC and there are probably other valid alternatives]