Is it possible to select all static units and object existing in a given area of scenario in FMB (a sort of multiple selection) and delete, copy or move the whole block to another scenario or to an other area of the same scenario ?

Yes there´s a way, but you need a little program: Group cloner or Monkey Wrench. You can get those in Mission 4 Today web or in Flying legends.

There is a way to do it in game as well, but you'll need to go the 'that other site'

Turns out, alt+left click and drag a box to multiple select is a feature already IN the game. It's just been disabled in the retail copy. You can re-enable it. You can also group rotate with it.