So I understand now. Mouse pass does work, but it sends the event up the tree (so to the parent, and to the parent's parent, up to the root). There does not seem to be a way to pass events to children or siblings directly (though you can do it manually by firing a signal that other objects are listening for).
In any case, there is a way to make it work. You just have to make the GUI elements that are behind parents of the ones below. This won't work for siblings, but in most cases this should be fine as long as you don't have many overlapping objects. I attached a sample project for you to see.
