The concept of the control is to put tiles in columns according to the rows that are set. This means that the entire layout and functionality are built around this logic and the tiles cannot be ordered in rows.
What I can suggest is waiting for the Q2 2013 release when the tiles will be available as separate controls, so you will be able to use them outside of a RadTileList and create the desired layout.
Scrolling can be disabled and more information on that is available here: http://www.telerik.com/help/aspnet-ajax/tilelist-scrolling.html
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed