No, it doesn't. Don't hesitate to do that missions any time you like, it mostly opens up the game even more.
I'm not far enough in the game myself to verify this, but I've read that the larger part of the main story is still happening after that mission.
The big change after that mission is :
You get your own castle, Skyhold instead of Haven as a base
Well, I made my way to Skyhold around the 20 hour mark, and that's when combat became a hell of a lot easier. Prior to that, I had to abuse the hell out of the tactical camera on some fights. Once I got specializations unlocked though, even Nightmare was a cakewalk. My setup was as follows: Sword and Board Champion Inquisitor, 2H Cassandra, Vivienne, and Archer Cole.
By making myself the tank, I was able to have much more control over the fight than I would by just leaving an A.I. to tank. The Tactical camera is broken, so I only used it to pace fights in the start of the game. Using the tactical camera past that was just redundant, since it glitches out constantly, and won't let you move your characters where you can actually move them. I noticed more often than not, if I moved a ranged character to a vantage point, and went into tac-cam, I would be kicked off of the vantage point because the game thought I shouldn't be allowed up there.
Knight-Enchanter is incredibly overpowered. More often than not, it seemed like I was just 2 manning harder fights with my K.E. and Champion. My Guard never dropped on my Warrior, I'd seldom lose barrier on Viv, Cole usually would stay alive unless it was a dragon fight, because I'd forget to move him during the wing attack. As for Cassandra, she would full-heal the party when she died, with a 60 second cool-down. That same buff also increases the parties damage and damage resistance as well, if I remember correctly. After I equipped her with a crafted piece with the masterwork to give a 15% chance to rez to 50% upon dying, I didn't have to ever rez her with Viv, because that "15%" was 100%. Seriously. She died a good 20 times over the course of one fight, and I didn't rez her a single time with my mage.
In hindsight, I think Solas would have been a better choice than an Archer, but I hate leaving doors unlocked. With Cass in my group, not only did I manage to absolutely destroy all demons/rifts, but when my party was low on health, I could switch to her, sacrifice her to get a full heal on the party, and she'd instantly rez to 50%, I'd say that's a pretty good trade-off. A few times, my tank was the only one left standing on a dragon fight. I would pop damage invulnerability, revive vivienne, get cole and cassandra both up, sacrifice Cassy once again to heal the group and get her to 50% instead of 10% from the revive, and go from there.
Seriously.. Templar and Knight Enchanter in the same party are so broken it's not even funny. By the 30 hour/level 12 mark I was going through Nightmare like it was Casual. I'd go through an entire play session without ever touching a potion. The hardest part about Nightmare difficulty, is getting to Skyhold. After Skyhold, it's a joke if you use a Knight/Enchanter PC or Vivienne, you can solo dragons. Seriously, I've done it.
EDIT: Also, remember, it's not after level 10 that you unlock specializations like some people think, it's as soon as you hit Skyhold. I just unlocked K.E. at level 9.
Best Answer
The difficulty does not affect the amount of experience or loot gained in Inquisition. Most say that playing on a harder difficulty, however, make it a more immersive and realist experience.