the phrase "not to mention that..." to me is a way of saying "I've more than made my point already. I don't need to mention this, but I'm going to anyways.
didn't feel it was worth getting a thread off topic. It is a strange phrase though.
I don't know the details, but apparently some side-quest you can only do in the beginning of the game triggers another side-quest at the end of the game that rewards you with Light Arrows...