Answer
Yes. Yes. The positive work done in separating the plates increases the electric potential energy stored in the capacitor.
Work Step by Step
A force is indeed required to increase the separation, because a positively-charged plate is pulled away from a negatively-charged plate. These charges attract, so a force is necessary, and it will do positive work on the system.
By equation 17-8, we see that the capacitance decreases because of the larger separation. Equation 17-7 shows us that the electric potential between the plates increases because the charge stays the same.
