Blood Elven forces were common throughout the Fourth War. As I said, their importance doesn't need to be justified.
It would be difficult for the Gnomish leader to be on the battleground considering he was in stasis by the way...but his people were there and they definitely had an impact.
And I note you actually can't prove that the ballistas definitely, one hundred percent came from the Silver Covenant. As such all you have is a theory with flimsy evidence. I have three or four theories, all with similarly flimsy evidence, but all of mine explain why there may have been ballistas but there were no exiles. Even one standing next to the ballista would have been proof. Zero though? Guess they forgot they were an important part of the alliance...