If there is no leak (FU+FOB=Block fuel?), then the safest config of the fuel system OEI is X-feed ON and all pumps ON. With only 750kgs on live engine side and X-feed OFF (shut), that fuel will disappear quickly in the event of a GA. A320 CEO fuel burn is 4T per hour per engine in TOGA so you will burn 400kgs in 6 mins!
As to landing with X-feed ON, nowhere does it say that you cannot do it. All that happens if you do is that the X-feed memo on the ECAM goes from green to amber as you pass through 1500' RA.
Hope this helps.
mcdhu