Going back to the early hours of the 7th, I think it's possible Janelle wasn't entirely forthcoming about why Suzie and Stacy left her house. I don't buy the overcrowding story, perhaps more something along the lines of some sort of tension between Suzie and Janelle - and Stacy, not wanting to let her friend go home alone, decided to go back to Sherrill's (as opposed to staying with Janelle, who had her boyfriend and a house full of relatives). The early calls might indicate Janelle didn't even sleep that night - after all, a lot of the grad parties were around her home. Maybe this argument was pretty heated, hence the reason the Kirby's had reportedly been so defensive with police/media outlets.
I think it's easy to characterize Janelle's behavior in those early interviews as cold and distant, and perhaps she did feel that way. I get the vibe from her that she was maybe jealous that the girls had gone off together, and when you're a teenager, those kind of things can play on your mind more than they should. I don't think she did herself any favors by acting in that way, and paired with her decisions upon discovering the house the way it was... it's easy to see why there's a lot of theories put into how she happened to be the first person to stumble across the crime scene.
My two cents.. Janelle calls many times, gets no answer - leaves some sh**ty voicemail(s), and after cooling down - she feels guilty. The sweeping up of the glass may have been a way to clear her conscience, maybe she was worried that Sherrill had heard these voicemails. When they discover the house is vacant, she deletes whatever voicemails she left. I think she and Mike probably snooped the house a bit - maybe even went into their purses (not staging) - and left. Why alarm bells weren't ringing between not just one, but two people? I wish I could answer that.. but it's easy to judge their actions from the outside.
Now, the phone calls are what I'm not sure of. I know they intially came by around noon, and again in the evening after the water park. Did they begin during the first visit to the home or when they went by the second time? I'm assuming it was in the evening, as Stacy's mom was around a little later and reported the same thing.