I could def see that maybe happening but two things come to mind that make it seem unlikely: first, since he was away he would have brought any drugs with him, even if he had them when they returned to Jacksonville Mariah would have been in bed before she could have gotten her hands on them; second, and I think this would be the more likely scenario, if Mariah did get into the heroin somehow, I don't think she would have been able to ingest enough to kill her, maybe make her sick but not kill her. Heroin is synthesized with some pretty nasty chemicals, I imagine it tastes abhorrent, so if a little one got it anywhere near her mouth she would immediately spit it out.
Of course, she could have gotten into different drugs but I don't think drugs had anything to do with her death. This is my reason why. Unfortunately, I have a niece who is heavily involved with drugs and I've met some of her friends, seen how they are about their drugs. They protect those things at all costs. If EK and KW were drug users, knowing their age (which is near my niece's age) and being some what familiar with some basic characteristics of drug users, I can't imagine them leaving drugs out to where Mariah could have gotten into them. Obviously it's a possibility but it doesn't seem to fit with what we currently know.
What do you think?