As far as I understand, the Sylvan elves aren't very friendly with the dwarves. And the Dungeon elves have a very bad, constantly hostile relationship with them. You can't really rely on Arantir; he's neither good nor bad, he maintains the BALANCE, and he doesn't have much else to do. So he might even interfere. Besides, he was killed in "Dark Messiah," if I'm not mistaken. And it's unlikely that everyone will suddenly decide to unite out of the blue during peacetime. So, I think that's a bit too optimistic.