But how to `modifie with the mouse` the object below ? For example : I select an object that is below an other, and after this want to move it; a click-drag on this selected area select and move the object at the top.
You can resize/rotate/skew the object that you selected by alt-click as usual with handles, and you can move it with arrow keys (I always use arrows for moving). However you are right that dragging such object with mouse won't work because it will switch the selection to the topmost object again. I think what we need is a new Alt+drag mode which would mean "drag what is selected now, without selecting anything else no matter where I start the drag." I will think about it some more and if I don't find any problems with this idea I'll implement it.