As I stated in another thread, if we're going with the theory that the Society has been a long-term enemy of Graham and family, then remember their love of the Xanatos Gambit. Long-term plans, deep-seated schemes. It's not too much of a stretch to speculate that while Mannanan was fond of collecting and slaughtering young boys, Alex was a special case, taken and subjected to a lifetime of torture specifically to hit Graham hard.
During that time, who knows what kind of dark spells, rituals, or other time bombs they put in him? The fact Alexander's turned out to have "the gift" for spellcasting himself just makes it worse. All Shadrack would have had to do with him was activate the trigger.
Rosella had no such trigger in place, so Shadrack would have had to strike hard and fast, then vanish. He just picked one hell of a time to do it.