I know who the SK is and I can prove it.

Summer Bethany Miles (Maewyn) aka the 8 year old is the SK.

"Perhaps if he hadn’t been distracted it would have been quiet enough to hear the killer coming. Perhaps if he had been standing he would have noticed the other person, moving carefully towards him."

These are the clues we have to the SK.

"Perhaps if he hadn’t been distracted it would have been quiet enough to hear the killer coming."

The 8 year old is always talking to her imaginary friend. "Occasionally she'd chat to the empty chair next to her, sometimes arguing and sometimes agreeing." If it was quiet Victor would have heard this.

"Perhaps if he had been standing he would have noticed the other person, moving carefully towards him."

8 year olds are small. If he were standing up, he would have been able to see the small child easier than if he were crouched down. Rather, if he were standing up he would be able to see her entire body moving because he would have a higher vantage point. Crouching down, his vantage point is lower in relation to her body. I have an illustration in case this is confusing.



In the illustration you can see that the only way there is a difference in what Victor would see is if the killer were short... like a child. If he were standing, he wouldn't see a grown adult until they were almost right next to him. If he were standing he would see a short child before they got as close. But when he is crouched he will see a child and an adult at the same distance.

It would not be hard for an 8 year old to strangle a grown man who was crouching with a piano wire. Piano wires are thin and you can't really pull them away from you.

As for the notes:

"As he slipped from consciousness, embracing darkness that was closing in around him, he barely heard a single piano chord over the rushing of his blood in his ears. This was a dark chord, a full chord, a chord of death and the last four notes poor Victor James would ever hear"

According to http://www.wordiq.com/definition/Chord

In music theory a chord is a number of pitches, usually at least three, sounded simultaneously (or nearly so). Some particular kinds of chord are named according to the notes of the scale that they contain.

The child could have been leaning or sitting on the piano to make the notes that were played. As he became unconscious the child would lean back and into the piano, striking the notes. I think it would be hard to play the piano in the conventional way and choke someone at the same time.

Those are the only clues we have. The 8 year old is SK.