This all makes me think that most of what we think is known about this couple is wrong.
I mean, what better way to cover up that you murdered a drug customer who wouldn't pay up or your girlfriend who's cheating on you, etc., than to convince the authorities that they're a foreign couple who's been assassinated for mysterious reasons? And really, a Coors Sebring race shirt sounds more like a southern guy than like a Canadian doctor's son.
If you discount the witness, is there any evidence that these two were a couple, beyond that they died together and might have eaten fruit ice cream together?
My guess is that the guy is from Florida (opposite direction of Canada), and if he's not a smoker but was carrying matches, he was probably into pot.
I would further guess that if they were together, they probably hooked up on the road, and if that's the case, she could be from anywhere.
Another speculation on my part: the witness appears to be covering up something, but he seems more afraid of being blamed for the murders than to be covering up actual guilt. Which sounds to me like he might have a connection to one of them.