I feel like the strongest argument that Durkon could have brought up is the fact that Redcloak has already won. The many injustices that goblins have suffered and probably will continue to suffer is, ultimately, water under the bridge. Nobody, let alone Durkon, could promise anything better for the goblins than what they've already secured themselves. The demand for equality as well, is pretty much secured, in the divine sense, in the form of the Dark One. Having their own god advocating for them among the pantheon of gods pretty is pretty much as equal as you're ever going to get, regardless of what the political situation among the mortal races is.
What neither party seems to have noticed though, is the fact that goblins were made as a persecuted race for a reason. In order for each god's clerics to have a viable source of experience in order to level up and better serve their god, they made Goblins and other creatures for the express purpose of being Exp fodder. Elevating the Goblin races out of this role just means the gods would have to create another race to serve the same role, but this time with goblins among the Persecuting races, essentially just repeating history. I don't think there's an pacifistic resolution to this problem, or it'd have been pursued already.