But we have no clue what they saw. Early reports said that the roommates could not get Xana's door open. They were calling and texting, but with no response (and could hear the phone ringing inside the room). They freaked out and called friends/family to come over.
It's not like they saw a big pool of blood. There's still doubt that either Dylan or Bethany went to the third floor. When police arrive, the two survivors are OUTSIDE and everyone standing outside knows about the deaths of Xana and Ethan (because the door was opened by some of the people who came over).
When the girls called their friends, they were probably still thinking Xana was simply unconscious. No one's mind would go to "murder scene" if they couldn't get inside the bedroom.
THe "impossible to rouse" refers, IMO, to them calling through the door and phoning.
So it's not clear at all how "obvious" everything was. I believe the "unconscious" person referred to by the young man who finally took over the 911 call was one of the survivors, who had fainted.
We do not know how many people went into the house. We do know that at least one person came right away (I believe there were 3, both within very close distances to the house) and helped open the door.
No matter how many people walked on top of the latent print, it was still there (and presumably was written in the blood of victims). Anyone else walking through pooled blood would leave non-latent prints (at least initially) and would give their shoe prints to investigators to be ruled out. It's not at all uncommon for other humans to be inside a murder scene, as many people are killed at home and their families are actually right there.