Does the presence of a prison camp contribute at all to the difficulty of conquering that enemy region, or does it simply offer a benefit to you once that region has been invaded?
To me the rules would say that it simply offers a benefit, but does not make the enemy region harder to invade. Is that correct?
Correct- it has no effect on actually conquering the territory. However, double-check the rules (mine are buried!) but I believe to actually "free" the POW camp requires 1 additional personnel unit to be discarded.
When free, the POW camp gives a permanent +1 to the personnel strength. So if you free all three, your personnel starts at strength 4.