It's probably cross of reading the King's Quest Companion and an overly-heavy dose of the AGD King's Quest working its dubious magic on my fanficcer's brain...but I've got theories.
As for why Alex was nailed first and why Shadrak didn't even have to touch him? The Black Cloak is VERY fond of Xanatos Gambits, and long-term schemes. Just check Alhazred and Mordak! Or Dahlia, if you want to go diving through the supplementary material for KQ1. If we're going with the idea that the Society is a nemesis for Graham and family and has been for a while, then the answer is simple.
While Mannanan has always been fond of young boys, enslaving them, and killing them off at the cusp of manhood (any further speculation on my part goes into Brain Bleach territory)...Alex wasn't an ordinary case. The Black Cloak had taken him specifically as an attack on Graham. Furthermore, Mannanan had his hooks on Alex for at least sixteen years. There are all kinds of escape clause curses, time bomb spells, dark rituals, and general bad juju they could have worked on him. So what if he had somehow managed the impossible and escaped Mannanan - they still could have him back under their control or dead at any time they wanted.
Rosella? If you want to work some of the usual twin tropes, they had a hook in her via Alex, but probably a weaker version. Shadrack had to go in for something quick and direct to activate it.
The content of the twins' dreams seems to reflect their lives and adventures. To use a DCU analogy, Alex would end up with a green ring of willpower, while Rosella is a blue ring of hope. Alexander literally went to hell and back, and it was treated very seriously while Ooga Booga Land wouldn't be out of place in a Tim Burton comedy. Tamir, despite Lolotte's hostile takeover, was a fairly welcoming and friendly place. The same could not be said for Llewdor. Did both of them risk their necks to save unfamiliar lands? You bet. Did they both endure some severe ordeals to do it? Sure. However, their approach is different.
Rosella acts, but she also inspires others to act. She did it twice with Edgar, first inspiring him to slip her the key to her cell and the second time making him break Malicia's brainwashing long enough to stop her. She gave Dr. Cadaver a spine - literally. She was also the one who inspired her own brother into finding her and himself by offering herself to the dragon. I'd expect Graham to spend a lot of time in her dream trying to convince people to help in some form.
Alex is better at direct action, at surviving. He's not as good at finding allies. No slouch, certainly...but it's not his element. He's better at trickery, as evidenced by his fake suicide in front of Shamir and his use of Bump-on-a-Log and Stick in the Mud's rivalry to get the swamp ooze. He also transmutes the dark magic of the spell books into good uses. Expect Graham's trip here to be full of unfriendly elements requiring an underhanded move or two.
As for the odd prophesy? Again, could be the AGD talking, but there was something in the King's Quest Companion stating that their universe isn't as stable as it seems. There's some kind of magic that created it when the sorcerers withdrew from the universe into the multiverse in order to save themselves. If the Black Cloak is undermining the very magic that keeps the KQ universe alive and separated from other realms, or there's a weakness in the magic the Black Cloak is exploiting, things could go very badly very quickly.
