There are multiple ways to do things. I wanted to be sure that I turned off the reading pane that same way that you did. Perhaps you used a VBS file to setup Outlook.
Anyway, I turned off the reading pane the same way that you did. It had no impact on the preview pane. (Not that I expected it to - but since you mentioned that you had it turned off - I figured it best to replicate your setup).
The only thing that I can suggest is the Office repair process. If you have already done that, then maybe others can help you out. At least we have documented the issue well enough.