Then what the fucking hell is it based on? Because it certainly looks like Excalibur, and Shirou does not have the ability to just make swords up out of thin air.
It is not for me to know what it is, but what it is not - that being Excalibur.
Erm, no, sorry. If I have an explanation and you do not, then you cannot realistically claim you are right, particularly when there is a hell of a lot of ambiguity, not to mention the possibility of an outright retcon which, as far as I am concerned, is not valid because it makes no damn sense.
It does not ipso facto require an explanation. It is sufficient to know what it is not.
Not really. If the only plausible explanation is that it is based-off Excalibur, then it is more likely that either Nasu ret-conned it later (which I consider invalid) or, else, that what he means by "cannot trace Excalibur" is that what he gets when he tries is, whilst
based on the original, so inferior that it cannot be called "Excalibur" any more.
You have yet to either refute any statement I have made
or provide a viable alternative, which means you have not got a leg to stand on.