In answer to no 3.
We have an off road rider in our area that has lost a leg just above the knee. He uses different crank lengths- The leg with the amputaion has a very short crank length but he is also a lot younger and stronger than we are. He basically rides one legged- with the other leg just putting in a bit of power.
but just a suggestion.
On Tandems- A lot of Family riders use cranks that have had an extra hole drilled into them for the pedals. This effectively will shorten the cranks so a very young short rider can pedal. Think it can put a crank down to about 140 length.
A quick search found these so it is possible to get a very short crank length- or it may be possible to convert your existing cranks.
http://www.sjscycles.co.uk/product-T...+-110-3079.htm