Question

when an object moves with constant velocity, does its average velocity during any time interval differ from its instataneous velocity at any instant?

Answer 1

For a constant-velocity object, the average and instantaneous are the same.  So the answer is no.  It's like taking a running average of a string of numbers that are all the same number.  The average is always the sum of the numbers divided by how many have accumulated, which will always equate to the repeated number.