Thanks Pieter. I'll try this and keep you aware of the resuslts...
QUESTION N°3 : How to add elements on the layout.
Well, I don't know a lot of things on .xml...But I've tried some changes.(advice :
always keep the original file somewhere for backup...). And so, comes the day when I've tried to add something to the layout : the media_front art.
Basically, I've cut/past the "box/flyer art" section, changed some informations and crossed my fingers...
IT WORKS, I'm a genius and I can do anything...but, the cart art is not where I wanted...Not a problem, let's change some numbers...
But, no, the cart don't move. I'm not a genius after all...So you'll guesse my question : how to add elements on the layout ?(I've already ask this on top of this reply, I know...)