I'll write a reply in a few by the way, but please take heed of one thing, Arthuria is nothing like the other Heroic Spirits. She gets a lot of privileges from being Typemoon's Golden Egg Goose.

Refer to this:

[center][youtube]https://www.youtube.com/watch?v=4zDt40QiY_4[/youtube]
[sup][i]Edit: Oops! Wrong video, now's the right one.[/i][/sup][/center]

Literally every Heroic Spirit regardless of time of existence knows Excalibur, thereby they all know Arthuria. You can't really compare her with any of the others.

Fate/Zero made a point of hammering how much of a special cookie Arthuria is and it's within the highest order of canon on the franchise.

As for your explanations, HSs itself can be related to some degree (as certain legends imply this) but if the legends itself don't mention it, it shouldn't work in world. You remember, that those stories are supposed to be real not invented here, despite what literature professor moogles might think IC, it1s just supposed to be a coincidence, except when it comes to weapons, which invariably all come from Gil's treasury.