(In reply to re(2): please read
by Ken Haley)
OK, Ady, I've found a failing case for your algorithm.
Suppose the audience choices are : C chooses 100, D chooses 20, E chooses 80, and F chooses 60. When arranged in sequence the audience members are DFEC, and the numbers are 20, 60, 80, 100. The only gap of 24 or more is between 20 and 60. DFEC is the 20th permutation of the table you said A should memorize (if it's not, change the audience selection to get to the 20th permuation, but I think this is it). So B finds card 40 and adds it to the original set of 4 cards, shuffles all 5 and gives them to A.
A now sorts the cards he's been given and sees that they are 20, 40, 60, 80, 100. All the gaps are equal. How is A supposed to know which card is the one B inserted?