Hi,
I am in the same situation as you. Negative balance exceeds 3x my initial money.
I tried to do 50/50 agreement but the they told me they are looking for a full amount and that I should deposit significant upfront payment this week.
I had the same position with other broker who covered my negative balance. So next time I open an account I will go for DUKASCOPY or OANDA.
I also found out that my order to sell half of the position at 1.2011 was not filled, but when I exported the historical quotes from IB, there was a high of 1.201135 that week so order should have been filled. That would significantly impact my position.
I will write a complaint to IB and I am going to find a lawyer who can help me.
I think all of us with negative balance should file a joint compaint.
Good luck