This is all based on the assumption that the cloaked man who appeared in front of Rosella was Shadrack....
I never stated such assumption, only saying if they are under the effect of the same spell, why are they (or /is he if the spell on both childeren is cast by the same person) casting it in different ways and why did it took quite a while to get a hold on Alex and not on Rosella?
As for the dramatic effect, wouldn't it even be more dramatic to have both cast in public? Also I imagine for the people of the Green Isle it has a greater effect to see their king, their saviour, beeing enchanted then the kings sister. So that argument I do not buy 
Alexander was raised, for all intent and purposes, by a wizard. One could argue that Alexander had a natural... tolerance for lack of a better word... when it comes to being on the receiving end of magic as a result of having spent so many years in the service of Manannon. This paticular advantage would not extend to Rosella, who was raised by her birth parents and was probably not exposed to the same degree of magic as Alexander.
As for the different spell castings, who can say if they are indeed different.
As for the big spectacle theory.... perhaps the purpose of the cloaked figure was to show Graham that his family
won't live happily ever after. If I were bent on revenge (and I believe that the cloaked figure acted out of a pure sense of vengeance by being so bold as to appear in front of Graham... as if to show how powerless Graham was to stop him/her/it), I can think of no greater way to set that revenge in motion than by attacking a parents child... in clear view of said parent.
Now I have not seen the overall story that Cesar, Katie, and crew have put together. I am merely conjecturing and thinking of theories here... just like all of you. Waltzdancing and I had many a conversation about who the mysterious cloaked figure in the Town Square is... and I view this conversation in much the same way. Do not take what I am saying as hints to the storyline to come or even attempts to steer people away from a paticular line of thought.
When it comes to the storyline of TSL... I am as much in the dark as all of you. I am saying this because I don't want anyone to think that because I have the Phoenix logo in my signature that I am somehow "in the know". I am not.