The evidence flags on the ground could be anything from shell casing to personal property.
As far as what ethnicity/race/background the people in the sketches maybe I would not take much from the sketches, since while some turn out to be accurate, they are very often not. That said I doubt the RCMP would have characterized one of the sketches as of a person who is"Caucasian/white" if they did not have other reason to believe so such as the witness directly saying so. Of course that could well be someone who is of mixed (sorry to use that strange term since all humans are "mixed", but it is useful here) native and European ancestry as most native Americans and many Canadian First People are. BC does have some Latin immigrants and persons of subSaharan African ancestry but not many. But odds are given the demographics it is just someone of European descent.
As far as the two cases being related, odds are extremely high they are but the investigators can't take that as a given.
Even if they are related, the facts released so far don't even exclude the possibility the two missing young men are the perps. I dont think that is likely but nothing excluded that. The guy who was seen stopped by the highway worker could have simply been p/o'd that he stopped and offered help, was declined which he thought was ungrateful and then gone on his way and someone else committed the murder.
My guess is the rcmp could have a likely cause of death of the guy two miles from the burned out camper by now. they also would what kind and caliber of firearm, from autopsy or shell casings left (and they almost always are left on the scene)