Author of the Antipodean Queen magical steampunk trilogy, and the Heest magical pirate trilogy for tweens (both published by OdysseyBooks.com), plus an increasing list of interactive fiction (listed at ifdb.tads.org).
The clues are spread very evenly so far. Penny has definitely aroused my suspicions, given her creepy hand-drawing (from before the party!) and Drew's insistence of her innocence. She also apparently went to the toilet several times which is right next to the staircase! And when asked later, she tells everyone Naomi wasn't upstairs ... why? However I'm wary of pointing my finger just yet when it feels directed. I'm interested that the staircase door seems to swing into the living room doorway - I'd assume from this that the door would usually be closed so it isn't obstructing living/dining room flow through, but then, wouldn't Drew and Hachi have noticed if anyone opened the door to go up or down the staircase? Or I suppose, not suspecting a murder, they didn't keep track.
The door suggests the killer was already upstairs - Adelle or Naomi. The way that Naomi seems to be everyone's best friend (Drew even saying 'she was everything to everybody') sounds very emotionally draining, and I wonder, was anyone that person for her? Maybe she was trying to be too many 'best friends' to help everyone around her and wasn't given that support herself, and nobody saw until too late (I also notice Naomi's dad is only mentioned once and in past tense? Dead? Grief could have something to do with it). I can't see any motive for Adelle, but it is possible at this stage.
Of course, if I am reading way to much into which direction a door opens, there are yet more suspects. Roshan and Laramie were absent for who knows how long to who knows where (Roshan gets very little detail ... suspiciously little). Even Drew's conviction that the only way into the the house was past him and Hachi is not entirely true - you could go through the garage ... or even a window ... I am hotly anticipating more clues, hopefully that could shed some light on motive.
At this point I have my suspicions about Penny too.
It sounds like her boyfriend pretty much ignored her when they arrived at the party, even though she was dressed to kill (ha!) I imagine that didn't go down too well with her, and I don't see why she would say that Naomi wasn't upstairs unless she had been upstairs herself. To give her the benefit of the doubt, I suppose she might have gone upstairs to use the bathroom if the bathroom downstairs was being used by one of the other friends.
However - gnomesw suggests that Drew and Hachi didn't see anyone go through the living room because the murderer was already upstairs. I'm wondering if perhaps Elan secretly went to Naomi's birthday party after all...