I think the best way to solve this would be to create a new Item element in your Scene, which is bigger than the scene (see also this post) and then put the MultiTouchArea inside this element. The width/height of the “wrapper” element can be calculated by the number of buttons you create.
I also think you should look into the Flickable element, which can handle scrolling by itself. This could make your code a bit easier 😉 (You could use a Flickable inside your scene which contains the buttons.)
If you do this, make sure to set the entityContainer of EntityManager to the correct container item to place the buttons into.