That's true. We've seen other Templars wearing them but never Cullen. 
But how would we know it's Cullen if he was wearing that helmet? Plus, it seemed like the templars in Kirkwall didn't wear helmets until the final battle.
As for the fledgling Inquisition, I think the plans were already in motion by the time that Cassandra questions Varric. I think this was a last ditch effort by trying to find Hawke and/or the Warden, as has been stated. I think by the time Cassandra questions Varric, the plans for the summit were already in place. As Leliana states, "It's in the Maker's hands now." So what I think happens after this, is Leliana and Cassandra take a boat from Kirkwall to Ferelden to join the summit. I still don't know how it is that Varric is there or decides to travel with them, (I'm assuming we'll be told in the opening, or somewhere near the start of the game.) However, I believe that Varric is there when the explosion happens because of the shot in the Fires Above trailer is him with bodies of those killed in the blast.
The thing I'm having trouble puzzling out, and I think we all are, is when Cullen leaves Kirkwall and where we meet up with him. I know we've talked about whether or not Cassandra questions him, and for me, even that is unknown. But he does leave at some point, but how does he get where? Does he, too, take a boat to Ferelden? Does he travel west and around? Questions, questions...