What are some competing arguments for acid rain?

There aren't competing arguments. There are several mechanisms for the release of sulphur dioxides and nitrogen oxides that contribute to acid rain, they aren't competing arguments... they just all happen and all contribute to acid rain.

Acid rain does not necessarily hurt the trees, it kills the mosses that thrive on the roots of the trees. The tree and the moss are an example of mutualism, one cannot live without the other, so if the moss is gone, the tree is gone.

It certainly weakens the trees and leaves them more suceptible to other threats.

Yes, but loss of moss is not the only factor that kills them either.