nested Layouts

When created, the 'Layout' object 'grabs' user input from the screen. What you'd need to do is just create a Layout object as and when you want it rather than trying to keep them in RAM all the time (it's also better for memory usage):

function showAMenu() {
  var AMenu = new Layout( {
   {type:"btn", label:"something", cb:l=>{}}
  });
}

var mainMenu = new Layout( {
  {type:"btn", label:"A", cb:l=>{showAMenu();}}
});

Also, i would like to change a value via the graphical menu, but without actually displaying the menu

I don't really understand what you're asking here... You can change the values in the object you pass to the menu and then I think E.showMenu returns and object that has a render function in it you can call to update.