Moving the backwater valve further down the line will just distribute the problem to other units when it is closed by a backflow condition, but why don't they also get flooded at this time if they are not protected by a valve. And since a BWV is actually a "shut off valve" when closed by a backup, any water used in the building has to overflow the lowest point in the system. If you put a valve on the toilet, the water could rise high enough to overflow the sinks, tub, and/or, shower. If we were there to see the actual conditions we might be able to design a system which would work during backflow conditions. And, I seriously doubt you have an 80-100 ft. "standpipe" in the yard because that would be a cesspool/drywell which would be completely illegal without an intervening septic tank to treat wastes, unless you are in "Georgia, Russia", or elsewhere which is not in the U.S.A.