Valerien : Just read your complex UI tutorial, and while it is functionnal, I think it is possible to open up a little:
The all buttons in one object while it works, can be really misused, not all buttons should be in one I think, but more functionnalities wise, they can be (every button relative to facebook in one object, every button relative to navigating menues could also be like that), so at the end you don't have to navigate as much into buttons (when you want a facebook functionnality, you know automatically which button it is, so you place it and you are good to go in that exemple), and you keep the advantage of not having all of them loading at once (even if this one is minor), it is IMO clearer than having to search every animation the unique object has to offer.
The UI elements as globals can be used, but the other way around (using a function to create it and set it at runtime, a decent use of containers to link the realtive elements between them) not only keeps the issue of the layer indexing at bay, but also helos during testing since you don t have to go into the layout that creates it each time you wanna check something. Also it is less risky since you wont have them present in every layout.
As for the UI layers, this is a tip I cannot agree more with, it is just the easiest way to deal wi a pause screen.
Keep it up, you are good