You might be bending your knees too far. Try bending them a little less, and later bend them more. Watch the video again so you can see how bent Master Lin's are during the different exercises.
For your feet, try removing your shoes and socks if they get too warm. It could be the energy moving. I don't think blood stops circulating in the legs during the exercises. When concerned, consult your doctor.
Standing for a long time is a matter of training. If you cannot stand through all of the exercises, sit for a couple of minutes. During this time, either focus on your lower dantian, or continue doing the exercises in the sitting position.