With all this I don't see the answer to which is the correct form "He rolled up his pant legs" or "he rolled up his pants legs."
The Corpus of Contemporary American English gives 130 hits for "pant legs" and 58 hits for "pants legs". So both are used by native speakers, and one is not hugely more common than the other. You can use whichever one you think sounds better.
It's a pair of pants not a pair of pant. A pair of pants, as in trousers (not trouser). A pant is something a dog does because it cannot sweat.