1. A global variable means it's global to the whole project.
A global variable set in event sheet 1 will be available in a later executed event sheet 2.
2. If you load a layout, it will load with every default elements you've setted it for. Meaning if you have a "Double jump" sprite that stands in the layout, you might very well destroy it in the first execution of the layout. If you load again the layout, the sprite will be spawned again.
You have to take care of such "disapearing" objects in the event "Start of layout" (with the help of a global variable for example).