I believe JW didn't witness the murder event b/c he and AS aren't congruent about Best Buy. JW's concern about surveillance at Best Buy indicates he had no familiarity with the place and would have been circumspect about setting foot on the premises. In contrast, AS was very comfortable with the area, he knew it's idiosyncracies from his first-hand experience and having repeated 20 - 40 minute sexual encounters with HML in the loading dock area.

I don't believe the "trunk pop" matters, it's symbolic of JW's knowledge HML had passed - that the deed was done, dramatic flair but otherwise insignificant. JW knew HML was gone when he first saw AS, "trunk pop" means HML is deceased, in JW's mind. That fact is pivotal, from that point forward JW could not have conjured last minute warnings or attemps to save HML, and he couldn't have turned back either. The deed was done.

JW's need to involve Jen and tell others his side of the story immediately indicates he didn't trust AS. Mutual distrust could have kept JW and AS hanging out together for a short while afterward but that couldn't have lasted too long.