They don’t fit onto the US 2″ ball but onto a 2″ receiver tube which I would have thought you could get made up for your car and I would prefer that over a ball fitment.
They do 2 and 4 bike versions and one side on version but they do say it won’t fit road bikes due to the way it hooks onto the fork which may be a problem for you.
As long as you stick to 4 bikes max I don’t see the weight being a problem, the moment arm might actually be less than a 4 bike set up as they stick a hole bike right out back.
If you have a bolt on tow ball something like this might work: http://www.4x4overlander.com/product/drop-plate-and-2-receiver-hitch/
You can get a 50mm ball to fit into the 2″ hitch but not sure of the legalities but if just for the bike rack then does that count as towing?
More bits here: http://www.abp-accessories.co.uk/product.php?cat=78
As for the lightboard – easy enough to attach to the bike wheels but would be nicer to have something on an arm that connects to the rack for less faffage but it removable for compact storage.
oh and it might be possible to find a receiver plate that fits behind the existing tow ball, or a combined tow ball 2″ receiver underneath so you can still use the tow ball for towing but have the receiver for bike racks.