Perhaps, but as far as I'm aware we're not going to Nevarra as far as I know. At least, not in this game.
I can see either Celene or Gaspard marrying a Ferelden if they thought it would help them take the throne. That puts Ferelden in a position of power as far as negotiations go, so they'd probably get to dictate quite a few of the terms. I really see it as being a true alliance between the two countries, unless you put a soft!Alistair on the throne. He probably wouldn't get as fair a shake as Hardened!Alistair or Anora.
Dictate terms? Extremely unlikely... As soon as the extremely blight weakened backwater starts trying to strongarm any negotiations they will be laughed out of the Talks. A "True alliance of two countries" would result exactly like it Always does when a small weak country marries into a Superpower, The Superpower will dominate the weak one.. Majority of collected trade and general tax revenues will go to the capital which will be in Val Royeaux to be certain, The heir will be raised in Val Royeaux and be part of Orlesian Culture since it is the dominating and "more sophisticated" of the two, the superior one. If Ferelden marries Orlais, Orlais will definitly make Ferelden its ******, The chances of this not happening in such a union are almost nil.
A political marriage between the two counttries if done correctly, would only help them both. It's certainly possible it would cause them to become a single nation, or ferelden as a vassal state of the orlesion empire, or just a permanant alliance in trade and warfare. You really don't know. The only time something like this is black and white is when one side has the other over a barrel. With ferelden regaining its stength after the blight and Orlais in a civil war it really looks like Ferelden would have the stronger stance in negotiations. They could very easily make a mutually beneficial trade alliance or mutual protection or w/e in exchange for eachothers help with thier own problems.
Nah, it would only help Orlais, It almost certainly would result in Ferelden being A vassal or Orlais or A single Orlesian nation. Ones A backwater and ones a superpower, its very clear this will be the result - its actually quite black and white. Even without a blight Orlais is vastly stronger than Ferelden, And considering they've had a blight.. 10 years is nowhere near enough time to recover from such an event.. Even with the Civil War Orlais will clearly be the more powerful of the two, Ferelden wont have any stronger A stance in the negotiations than they normally would...Which is itself A weak stance anyway, but since they are nowhere near recovered from the blight it will be even weaker.
Mutually beneficial Alliance is extremely unlikely, after the Fereldens fail trying to dethrone their stupid Monarch for marrying Orlais, they will be pressed into war with Nevarra and helping their overlords expand.