Sorry, i did not read (understand) the whole question - english is not my motherlanguage
Yes, you only can make an invisible border, so the player can not "fly underneath the HUD".
You can solve the center-scroll-problem, when you calculate for example: scroll to LayoutWidth / 2 - (HUD-Element.Width)
So the center-point is the half of the layout width minus the half of the width of the HUD-Element (width of the graphic).
Sorry for my english and i hope, you understand what i mean and it could help