Is there any way to turn off the automatically opening window for a preview in Marsedit?
Yes - this is a per blog preference.

I found this on the Mars Edit support page...


Double click the blog in the main window to open settings.
Click the "Editing" pane.
Find the "Preview Defaults" at the bottom of that section.


Here's a picture showing that you can choose to have it off or remember the last state as well as the default option of opening.