[x] Give more arguments why the Harsh Mountain should reject the offer.
-[x] Appeal to the Harsh Mountains pride and point out that he will probably have to be subservient to Skerhogis in such alliance and make a counter offer that if Skerhogis is defeated he will be entitled to the power and fame that comes from it among mortals.
-[x] Give concession of shouldering restoration of his herald and promise further help in regaining his mobility. Make a point that despite everything you are God of your word and will respect the deal opposed to certain snake.
I do not particularly like the first two points. Skerhogis may be hungry, but he is hardly mad, and has proved himself smart enough to win an engagement against our alliance. His approach to negotiation and battle also shows him a careful and skilled planner. One probably wouldn't mind losing themselves to madness if it gave them a clarity of mind to outwit "sane" opposition.
Similarly, he didn't offer us an alliance against the Mountain, or the Mountain against us; he simply offered a place by his side to whoever is willing to submit. Naturally, the one who doesn't will have to be terminated, possibly as a test of loyalty, but there is nothing inherently unfair in the offer, and I am sure he'd be just as glad to see us both bend the knee.
The point about subservience stands, though. Once forced into a position of a vassal, you are at the mercy of your sovereign, and can do little should they alter the deal or demand something you'd rather not give. Though knowing how Skerhogis operates, he has little reason to do so as long as the terms of alliance are observed.
Harsh Mountain may get his freedom of movement, but will lose the freedom of action. Unless he joins Skerhogis for good, the latter will come after the other spirits' followers eventually. It'd be exchanging one leash on another, and that one would be harder to get out of.