Regarding the standing position, did you have much trouble getting over having your elbows on your thighs? That's how I learned, and switching seems like it would be difficult. What was your experience? Did you teach yourself or have coaching?
PS - Nice crackle glaze in your photo. Your own recipe or commercially available? I love crackles.
Nope, no problems switching over to the standing position. I just tuck my left elbow into my side when centering. I think I use my arm muscles a bit more now though. I use the foot pedal with a length of redi-rod screwed into the top of it (wheel height), with a knob on the top. (foot pedal is on the floor and I use my hand to control speed. I don't alter the speed as much as when I used it as a foot pedal) Having the wheel head about an inch below my belly button works for me. Good shoes and mat to stand on help too when you are on your feet all day.
I definitely would follow John's advice about getting proper medical advice re positioning when working.
(my avatar is actually dried mud flats)