can shirou trace a lightsaber?
Yes. The handle is the main part that produces the laser. Once he races that he would have his own light saber. However I believe magical swords are far superior than light sabers, because they are able to block laser with magic and have other magical abilities.
He probably can't for the same reason he can't trace ea. He can't trace things of outer origin and saber crystals are pretty much that. Without a crystal a lightsaber is just a rod of hot plasma.
No. He cannot trace Gilgamesh's Ea because it was made from something not from Earth. Likewise, a lightsaber is made from materials gathered from outer space and built using a technology that cannot be understood by an ordinary Highschool boy
Saber is pretty light, could he trace her?
Why doesn't Shirou just trace the key that opens the Gate of Babylon?