Re: Seiza and Calves Pain
Seiza is a difficult position anatomically for the knee joint and can lead to somewhat restricted circulation in the calf. On e way to combat this is to warm up the calf muscle thoroughly (a few hops or jumps) before class and it might be why its not so painful to sit for you during class.
Another issue is make sure that you are properly hydrated; again to maintain circulation in the lower leg, prior to practise.
P.S. Seiza never becomes "comfortable" just less excruciatingly painful