There's nothing in the game that states there's more than one temple intact enough to be the home of any Sentinels.
There was nothing there stating that there was a nearly intact Temple Of Mythal in the game, up until we actually went there, so it's not really a good argument. We saw in Trespasser how many undiscovered ancient elvhen ruins there are, as well as a myriad of Crossroad-like pocket dimensions, and there's no telling how many of them exist yet and which are inhabited. Solas wouldn't be saying to Abelas that "his people yet linger" if he didn't mean it. He may be a trickster who deceived everyone by simple omission, but he hardly ever directly lies.
Also - if I recall, Leliana's reports mention strange elves that look like Sentinels, NOT necessarily Sentinels themselves. Solas isn't one, yet he wears similar armor, meaning that they were probably "in fashion" across Elvenhan before the empire went bust, lol.
Plus, the game we can play in DA Keep mentions Serault inhabitants being sometimes raided by strange elves wearing red vallaslin. None of the elves in Temple of Mythal wore red tattoos - in fact, if I recall correctly, they were all emerald green.
Given that we know that Fen'Harel detests vallaslins for what they represent, and went about to free people from them, and effecivly from slavery to others, it's unlikely he'd force anyone to wear it. The guy stands by his principles... yet.
And red? I'd like to point out that red is a color of the Blight and Solas detests and fears Blight seemingly more than anything else. Whoever resides at Tirashan, I'd be more inclined to believe that they're tied to the trapped Evanuris more than they're to Fen'Harel.
That would be your imagination. And people walk "all the way there" all the time. For instance the Cole war table mission "Locate Rhys and Evangeline" is acquired on one side of the map and then the follow up mission "Deploy Rhys and Evangeline" is on the other side of the map.
That doesn't mean anything at all. The Inquisition at the height of its power has many agents and allies, and messages can be delivered quickly across the map via message birds - plus, we have no idea of the time-frame each mission gets to be accomplished, realistically in-game, other than it's definitely more than a few days - and realistically, few weeks or months even.
It is much more likely that the group of elves we're already invested in - the modern elves - is the group Solas is concerned about. Solas made the elves mortal, thus the ancient elves have all but died out. The "modern" elves are the people we've seen suffering, treated as second class citizens, seen as slaves. It makes much more sense that Bioware, having invested us in the plight of these elves, further explores the future of these people as possibly being restored to their natural state and having a shot at rebuilding Arlathan.
Solas isn't concerned about modern elves any more than he is concerned about others, since many more groups other than elves suffer greatly and the whole world suffers because of decisions he's made in the past - in fact, after Halamshiral, he can be caught off-guard and admit that he feels more connection with the mages than modern elves. Because this is exactly what cost the world when he created the Veil to save everyone from Evanuris and their mad plans: he cut most people's conscious connection with the Fade, and only mages in modern world retain it, at least in some measure.
He may value the remnants of ancient elves for their memories of how things were, the lost knowledge and power and likely would seek to rescue it for the sake of these memories or knowledge, but ultimately he wants to return "the world of the elves" (the world with the Fade flowing freely through the world and the people connected to it, as they were before), NOT necessarily ancient Arlathan or Elvenhan. He tells Dorian that Elvenhan was no more innocent than Tevinter in its time - a harsh assessment, given his sentiments about Tevinter. He also tells Dorian not to romanticize ancient elves; that it is pointless.
Plus, don't forget that if Inquisitor doesn't complete the rites in Temple Of Mythal and doesnt' ally with Sentinels, he's very upset, but he kills them anyway - in fact he seems no more upset than he was with conscripting mages, keeping Wardens or sacrificing Bull's Chargers.