I also noticed that Alric's dream is the DISPERSAL dream, and all of the SHADE's dreams are also dispersal dreams. Hmmm...
I agree with your idea about Sinnis. Some past enmity between Alric and the shade is obviously present, perhaps Sinnis defected to the Fallen?
Or perhaps one of the avatara was always a betrayer, just waiting for a time to indulge his dark powers?
Phelot- The shade behind the massacre at Avon's Grove
Herod- A shade from Myth2...
Mazzarin- A shade turned to the dark by the Fallen... killed in the TFL mission "The Watcher"
Sinnis- Shade confronting Alric in Myth TFL
The shades were the magic rank of the Fallen's armies. Perhaps their knowledge of the Nine aided the Fallen in some way?
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As for the head, I'll say one thing:
DEUS EX MACHINA