[Turtle91]

As far as I understand it, you can install on any device under the same appleID without re-purchasing it.

To lock the orientation: tap the Aa when you have a book open, then select the second tab (at the bottom) and you have a choice: Allow device rotation (which goes off of your device settings), Lock portrait, and Lock landscape. There are also some other options if you scroll a little further down that page where you can select Paginate, or vertical scroll mode, and also select how many columns you'd like Marvin to use in either portrait or landscape modes.

edit: Also, when you download Marvin 3 you have a fully working version - all the bells and whistles work. I think the only difference in the paid version is removing the ad banner at the bottom, and allowing multiple books in your library at one time.