In response to your argument regarding sensory organs: No that doesn't make sense, my argument against parallel universes is that to experience something in a parallel universe while sleeping we would need some type of sensory organ to relay that information back to our brain. Such a thing does not exist. Would this mean dreams have to be internally generated?
In response to your brain stem argument: This argument sounds promising, how would we see the affect if parallel universes acted on our brain stems if it is even possible?