Say Up is positive. When the up elevator sees the separation increasing at a speed of 10 m/s it means that you have +5 - - 5 true to the observer below. He sees each of them moving at the same speed away from one another. Answer A.
Problem 6
6 looks right to me. A timer would be the second best answer but it starts at 0. Answer C
Problem 7
It can't be zero. Nothing would get anywhere if it was zero. When you look up and see a plane, isn't it going in the direction you think it's going? It can't appear to be going west. That would take it to Vietnam or Melbourne. It must be flying east when seen flying from ground level and since we are not asked to consider air resistance or the earth's rotation, the answer must be D.
Problem 8
I'll go with your definition.
Problem 9
It's moving forward as well, just not as quickly. 126 - 68 = 58 m/s if 126 is taken as positive. Answer D
