"It is not entirely clear who is going to obtain all the information necessary to determine whether an individual appears to be eligible for a credit on the exchange, to confirm after the year ends that he actually was eligible for subsidies, and to confirm whether employers are actually liable for any penalties (commencing in 2015). Nor is it clear how they are going to obtain this information.... [A]bsent more guidance, providing too much information in Part B could lead to inappropriate conclusions."