I could see him conquering post Blight/Breach Ferelden despite the Civil War/Breach unless other nations stepped in to protect Ferelden. Nevarra or Tevinter wouldn't be feasible tho unless he got considerable support from other nations.
I don't think that Ferelden would need protection from Orlais. Orlais may be powerful, but don't forget that they managed to kick them out once. They were occupied for something like 80 years before doing so, but they did it! The thing with Ferelden is, no matter who you put on the throne in DA:O, you basically get a strong leader. The weakest possibility is a softened Alistair, which doesn't seem to have an impact on what happens in the country to any noticeable effect. Sure, Ferelden is still weak from the Blight, but it has been 10 years and that's a lot of time for a whole new crop of soldiers to have been recruited and trained. The way I see it, Orlais trying to conquer Ferelden would give Orlais a challenge no matter what. Plus, for me personally, Loghain was a Warden and still alive, so the Alistair/Anora Ferelden could call him back for help if they needed it. Not to mention that all of this is based on choosing Gaspard, which I did not do (I'm a Celene/Briala reunited person, but that's because I read the books), so I don't need to worry about this, haha. I like Gaspard as a person, and I wish that I could have had him imprisoned and reunited Celene and Briala, but I wanted them together more than him alive.