Actually in France you can't control most ISP-supplied consumer routers.
And for sure on this one you don't have the control at all.
Source: i'm french.
Of course you can.
You can even use your own router.
No you can't use your own router instead of the freebox.
Well you can, but you'll have degraded performance and not access to tv/phone services.
Well, you can, but you lose a whole bunch of important functionality, like being able to watch TV over IPTV (source - I work on SFR's STB, and we watch the competition closely :D)