Again, this is false, obviously so.
I don't think so. DarkStar's reply makes perfect sense, maybe re-read it again =)
Taking your example: "Suppose there are two investors, A and B, and each have 50 btc onsite and 50 btc offsite. A is lying about it but B isn't."
A 50 BTC loss to A is 100% loss of their bankroll, but a 50 BTC loss to B is only a 50% loss. This drastically changes things as a margin call to A is fatal, while B would just be looking at replenishing their onsite investment.