Re: Seiza and Calves Pain
If I may...
Yes, you may find seiza more uncomfortable if you get too much blood pumping in your legs prior to adopting the posture. This may be why you are finding it more difficult toward the end of class - along with other reasons. So stretch the legs (front and back, lower and upper) and the back but don't get the limbs in question too pumped.
What I always tell my students, aside from the usual "you must give yourself time" (which is accurate and really the most relevant solution here), is that they can do two more things: a. before you adopt the posture in class, while on your knees, be sure to stick your thumbs into the back of your knees, grab as much lower leg muscle as you can and roll them outward - then sit down (being sure to not sit on your feet - cross your big toes at the most, we do right big toe over left big toe); and b. get yourself a zafu and use that as your main seating furniture at home. Spend your leisure time (e.g. video game viewing, tv watching, etc.) and/or your study or work time, etc. sitting in seiza on the zafu as much as you can. Since you are not actually sitting on your feet you will find that you can sit longer - which will help your body more gently acclamate itself to the posture. (When sitting on a zafu in seiza - remember - if your back starts to hurt you are sitting too far back on the cushion; if your knees are starting to hurt, you are sitting too far forward on the cushion. Note: a zabuton makes things kinder on your knees.)
Hope that helps,