FYI - The holy grail of the Fuyuki wars isn't the actual holy grail. Moreover, it's more of a <s>spirtual existence</s>ritual than any sort of physical object.
The grail, both the greater and lesser, are the Einzbern Homonculi (in reality, their hearts function as the container - or at least in the case of Ilya), but that's only really the physical side of the case. The 'real' grail is the function of punching a hole through reality to reach Akasha, which involves a specific location (Fuyuki), container (Homonculus) and energy enough to make that hole (Servants). In other words, it's not any one thing, and most of those things aren't objects.
So, no. Shirou can't trace the holy grail (the Fuyuki holy grail. Maaaaybe possible if it were the real one, but I don't think it would function the way you think it would).
There's lots of stuff that you're saying that is off, but I'll just break it down like this:
The function of the Fuyuki grail war was to revive
the Third. The whole wish business is basically a lie made by the three families after the first war (technically, the grail can grant wishes, but it wasn't created for that purpose, at least outside the wish of reviving the Third).
Function of the grail.
A bastardized Third Magic is used to make Servants, but it's their own power, and it doesn't really have anything to do with how powerful the Third magic is. Blah, blah, blah. The Servants power doesn't have anything to do with the Third beyond giving them bodies (which are not actually 'meat', but spiritual ones. They're technically 'ghosts').
Unlimited Blade Works is a Reality Marble.
Look it up. If you're seriously asking where it came from, you should really look into what a RM
is first.
EDIT:
IF the grail were a physical object, I'd say that Shirou
could actually trace it.
The
real problem is that there's no way in hell that Shirou could possibly power the thing. It takes the power of at least
six Servants to power the grail, and the function of it is only to make a hole in reality to Akasha, which is the real stuff that allows for wishes and crap.
In other words, even if he could, he'd need do go through a holy grail war of his own just to use the damn thing.
But that's all moot, since it's not a really a physical object in the strictest sense.