I didn't see the graverobbing as that big of a deal for the desecration part, partly it read to me as more of someone digging up a grave to steal a necklace or something, rather than to steal a body part. If Murtagh took Oromis's head or something though, that would have been really messed up.

How did the werecat know, is my question? It's not like people were dropping dragon scales into the water.

Second, Thorn could've literally dipped his tail in there, and then flung the fish out of the water.

Also, if the grave robbing quest was eliminated, and Thorn did the above, the book would've been shorter, to no detriment.