LN has no desire to help Aurvandil. We'd be perfectly happy to help you destroy them.
There is just one thing standing in our way, and that's you, D'Hara. If you would give us peace, we'd clean up LV quite quickly, then join in the war against Aurvandil.
So, no, LN is not the de-facto ally of Aurvandil right now. It's D'Hara, who is doing everything they can to prevent LN from joining the war on the side of the north.
Basically, Chénier, you really need to look at it this way:
Which is more important to you: keeping LV alive—a realm that's only been your ally for a short time, that you can't actually keep alive anyway—or seeing Aurvandil destroyed?