I don't think that making a Heroic Spirit has to be so cut and dry. Type Moon took [b]A[/b] representation but those aren't necessarily the only ones. You say Arthur's best known artifacts are Excalibur and Avalon, nobody ever said Excalibur had the power to level buildings. You could take an entirely different approach to it. Such as, It was known as "The sword which sliced through iron as through wood." This could easily be interpreted as a Noble Phantasm which would nullify any armor, magic and Noble Phantasm alike. Which would bring Arthur and his opponent into a dual of skill, which would certainly fit with the fact he is a knight. 

Also, I still do not believe a player's version of a Heroic Spirit [b]has[/b] to coexist with Type Moon's version.

My point isn't only meant with Arthur, I think the same for all servants that have already been used. Arthur just seems to be a prime example.