If a person in the elevator were standing on a bathroom scale calibrated in newtons, what would the scale read while the elevator was (a)...

I assume this question relates to the common example of the equvalence priciple of general relativity.  The idea is that if you are in a non-inertial frame (i.e. accelerating) then it would be equivalent to being in a gravitational field of the same acceleration, so if you were in an elevator outside of any gravitational field that was being pulled UPWARD at an acceleration equal to that due to gravity on Earth, then it would feel as though you were on Earth.  To answer yor question:

 I assume you mean an elevator outside of any gravitational field.

(a) If you are moving at constant speed in any direction, then you are in an inertial frame, which would be equivalent to zero gravity so the scale would read zero.

(b) Slowing to a stop from your downward motion is an upward acceleration, so this time it is the same as being in a gravity field.  The scale would read the force that your body exerts on the scale, which according to Newton's second law is F=ma, the product of your mass and the acceleration of the elevator.

It's interesting to note that because both you and the scale have mass, they produce their own gravitational field and so will always attract each other, but this force is exceedingly weak and would be unnoticable.

Hope this helps.