1. At the moment it's restricted to NotePlan's folder in your documents folder in "On my iPhone/iPad". This is due to the sync and having different external folders on iOS and Mac. I'm not sure how this should be handled. But I plan something around that.

2. There is no separate preview, but most of the markdown is being rendered, so you don't see it. To be specific: Headings and tasks are rendered properly without markdown characters. But tables are not supported yet.