I have been examining Alix's neck very closely, even with a magnifying glass, on the few photos were her neck is bare, and I have never noticed anything that even reminded of a scar. It must have been visible, if the brits were so concerned about it, but there's nothing to see.
I'm beginning to believe that the scar was just a rumor, started by the ones who was against a marriage between Alix and Bertie.