That, in theory, about the blood flow, could possibly be true. I'd say the blood spatter analyst would be able to tell more about that. Being shot in the head though, at what I'm guessing would be close range, the smaller entry point would not bleed out as much, while the larger exit wound, where the blood, and other matter, would come from, would probably be absorbed by the bedding. (just guessing)
A little more pondering. The Rhodens were not afraid to fight. I believe if they were awake, they'd have fought tooth and toenail against their assailants.This would have led to defensive wounds, and shots to other parts of their bodies, like CR1. I think they either woke CR1 up as they came in, or he was not yet asleep, and heard them when they came in, and he just couldn't get to his firearm in time. CR1 was a snafu in their plan, but they handled it and moved on. CR1, KR, FR, and GR, do not strike me as being shrinking violets. I don't think they'd have just laid there, pretended to be asleep, and let them shoot them, and their family members, in the head. Especially being parents in the home with children, and not knowing if their children would be next. I'd also not be surprised if DR, CR2, HG, and HR, knew how to handle a firearm too, or had a baseball bat nearby. If the shots were anywhere else, other than only to the head, then I could see more along the lines that they were awake. They may have awakened for just a few moments of realization that something was very wrong, but not in time to fight back. Then again, I could be wrong.