The low breakup fee in Special Case 2 suggests that the companies think a material adverse change in tax laws before deal closing is fairly likely.
I don't agree.
Treasury is doing all they can without new statutes and those are not going to happen until 2017 at the earliest and more likely never. By then the deal will be closed.
The reason for the low breakup fee in Case 2 is that the cost would be very high and the parties recognize that.