I thought that Josephine gave sound advice, and I go along with her on it. King Markus is old with waning power, so this will be a one sided alliance, as someone mentioned earlier in the thread. Also, I consider that I've already helped Markus by completing the war table mission The Serpent of Nevarra, so I can afford to overlook him in favor of Tevinter.
You are given numerous opportunities to deal with Tevinter throughout the game, mostly through war table operations. I typically try to be "friendly" toward Tevinter because my goal is to show that they are not my enemy, Corypheus and his minions are, and if you're willing to help, or at least won't stand in my way, then we have no problems.
To that end I...
* Lend Magister Tilani political influence through Josephine rather than use threats.
* Allow Cullen to send "real" templars to deal with the attackers. The result is humorous, and also shows them we aren't f-ing around.
* Do send Movran north to gnat Tevinter, but then leave the Avvar to their own devices.
* Allow Josephine to bribe the Grand Archivist with gifts rather than steal or threaten.
* Follow Leliana's suggestion and send House Amladaris the information as a gesture of goodwill.
* Have the pleasure of letting the nerds at the Minrathous Circle geek out over translating the ancient texts I discovered. [Yes, I know there are no options for this one, but I still count it as showing the Inquisition's worth.]
* Allow both Servis and Alexius to live and recruit them to work for the Inquisition. I believe Alexius is no longer a threat and Servis was hired by a third party and is not loyal to Corpypheus, so I feel that not using them is a waste. They each have valuable information and contacts in the Imperium to make use of.
* Stop a war between Tevinter and Nevarra.
Back to this mission... For me there are some pros to helping Tevinter that don't come into play if you help Nevarra. I believe that having Tevinter as a friend, or at least as a non-enemy, is far better than the reverse. I like the idea of showing my strength without posturing, because that would just cause more Magisters to turn to the Venatori. This is a chance to show Inquisition military power near the Tevinter borders without directly threatening Tevinter itself, and you'd better believe they'll be watching. Siding with the archon against the Venatori sends a message to the Venatori as well as their supporters within Tevinter. It also makes a tentative ally of the archon himself.
We're all using each other and we all know it, but as long as the Venatori and Corypheus are on the losing side I'm fine with that.
And far down the line of considerations -- I won't pretend it's not in my mind at all -- if my dealings with Tevinter happen to benefit my boyfriend some time in the future if or when he returns to Tevinter, then that is also a pro.
But here's the thing... even the magisters who oppose the Venatori still have no qualms about performing human sacrifices.
Even Dorian admits those who are against such practices are the minority in Tevinter. There obviously are exceptions, but as a whole they're disgusting.
So you think, "your mind is made up, then".
Not really. I don't want my character to seem like a hypocrite after telling Dorian she didn't care about the bad rep the Inquisition was getting somewhere for associating itself with a Tevinter mage.