Hmm... That's an interesting theory, to say the least. However, one thing bother me with all the talk about the sword: The servants are spiritual beings, and the physical body they have in order to fight and pose nude to their masters is a temporal convenience. Well, you could say our bodies are such as well, if you believe in a soul that is kind of separate from the body. However, Excalibur is a piece of metal with some magical abilities. It existed before Arthur and perhaps in some form remained after him.
Dunno. What I'm trying to say is that the sword shouldn't be in such a way linked to Saber, or it shouldn't be conveniently materialized as if it was something that happened to be misplaced during the initial spawning of Saber. And furthermore Saber hasn't indicated once that she's missing something, like the original Excalibur, and is instead using some makedo.
So, if the missing Excalibur if not just a symbol for the missing link between Shirou and Saber, it shouldn't exist as available to those two. It's not part of the story. Unless all the other Master/Servant pairs also can have external mystic weapons or gear.