To bring up a pause menu, make a new layer and set the parallax to 0,0. Parallax determines how fast or slow the layer scrolls relative to the others; 100,100 is normal, 150,150 is 1.5 times faster, 50,50 is half as slow, and 0,0 means it won't move at all. Which means no matter where you are in the level, the pause menu will ALWAYS be in the center of the screen.
As for loading new layouts and then trying to go back to the previous one... yeahhh, don't even try. You'd have to manually save the state of the layout before leaving it and then manually reset it once you go back. Pause menus can be inside the regular layout, just invisible; when the player pauses, make the menu visible. No need to switch layouts.