The greatest strength of the Qun is that everyone has a role and purpose, there are no wasted bodies.
A nation that retreats doesn't equal a conquered nation. I wouldn't hold much stock in a treaty either. You know why? Because a treaty only lasts as long as for one side to re-arm/regroup and to try again, (obviously one side would't break a treaty unless they thought they have a great advantage(s) that would led to a high chance of victory. Breaking a treaty and then losing, usually leds to total submission.)
While the human lands are under siege from Veil Tears, Mage-Templar War, and the Chantry in shambles as well. (also, with a rebuilding Ferelden due to the blight.) The human nations aren't in a position of strength to unite against another force. The Qunari have natural allies with elves (the enemy of my enemy is my friend.) If I was the Arishok, I'd head for Orzammar and broker with the Dwarves. With the agreement being that the Dwarves stop all dealings with the lyrium trade to the Chantry and deal with the qunari instead, (best time to do it with the Chantry in flames, also crippling The Chantry's Templars.) Ensuring that it would be in their best monetary interests to deal with the qunari rather than the Chantry, (the Qunari could open trade with some of their exclusive items, clothing, weapons, whatever, to entice the noble caste.) Think of the things that the Qunari could develop with lyrium. In exchange, the Qunari would aid the Dwarves in the Deep Roads, (in addition to getting your foot in the door to have The Qun around the casteless...)
The Chantry would have to march to Orzammar out of desperation, (into a highly defensiveable postion,) but it wouldn't be in their best interest to decimate the dwarves because a) If they did, then they would have to have to mine the lyrium themselves, even though it is dangerous to all races, it is less dangerous to the dwarves, and (B)then they would be responsible for dealing with the darkspawn.
With the Qunari's superior Navy, they would have an easier time than the other nations controlling the seas. Forcing the humans to travel inland, which would be more time consuming, (if the Qunari could get an alliance with some of the Dalish clans, they could provide guerrilla tactics against a traveling army.) If the Qunari could defeat the Chantry there, they could leverage lyrium against the Chantry in exchange for the Chantry teaching them the tools of the trade of being a Templar. Which then they could use on there most hated enemy, Tevinter.
Downside for the Qunari is that Mages would have time to organize if they were able to do so. Even though the Qunari are easily the hardest on Mages, it isn't like the humans are sympathetic to Mages on the whole anyways. With the Mage-Templar War taking place, the tension towards Magic may be at an all time high since Andraste.