Question

determine the power consumption under uniform acceleration

Answer 1

The first formula P=½Fat is correct.

You are right about the velocity thing. The problem is, that you are inserting v = at in the last formula. But, v=at is the *final* velocity.

Forget about instantaneous and average velocity, that will just confuse you (well, it confuses me. Instead, just remember that the velocity is not constant, only the acceleration is.