Because she let Solas do most of the talking with Abelas, I don't believe she ever got the "not of my people" speech directed at her, otherwise it might have given her pause for thought. With my male Lavellan it is striking that Abelas refers to him as "not of my people" and "Shem" (which I do not believe is a mistake by the writers but intentional by the character), yet he says to Solas during the discussion about what Abelas should do next "Elvhanen such as you.". Of course everyone is concentrating on the well at the time but it is pretty obvious that Abelas does recognise Solas as one of his people. He could explain away knowing the language so well by his travels in the Fade but not that sort of recognition, which is why the conversation about leaving comes straight after we get back to Skyhold: he knows sooner or later people are going to look more closely at what was said at the temple and awkward questions will follow.
Small point but Dalish are fluent in elven, too. It's not as obvious because for gameplay reasons we always hear Dalish speaking perfect English (common tongue, in Thedosian terms), but according to the lore, Dalish speak elven among themselves. I don't think it's clear that Abelas recognizes Solas as an ancient elf, though he might suspect soemthing. So I don't see Solas' actions afterward as resulting from realizing that the jig is up. What I do think he's contemplating is the fact that he now realizes that Mythal is alive and active. He seems shocked that the sanctuary has been preserved. I see this as the reason that he realizes he has to either embrace his purpose fully, or in the case of romanced Lavellan, give it up entirely.
It is all very well the writer saying that he was intending to tell her the truth; we don't know that in game, she doesn't know that. All we have is him skirting the truth time and again, then outright deception in their last ever speech together before they leave for the final battle.
When I first met Solas I thought he was going to prove my ideal romance but sadly that was not the case and the more I think about it, the further he falls in my estimation. Sorry, Solas fans, but Dorian has supplanted him in my affections.
That's fine and not everyone has to like the same things. I tried to like the Cullen romance, but the fact that nothing unexpected happens and it all ties up neatly and happily just left me bored. I think the Dorian romance would do the same. With Solas, the end hasn't been written yet, so I'm going to reserve final judgement on it until I've seen how the writers wrap it up. That in itself might be enough to put some people off the romance. Me, well- it hooked me badly. *grumbles*
BTW I get the sense from some comments that people think we're minimizing the deception and cruelty of what happens, and I don't think we are. It's just that I believe Solas has reasons for what he does and they aren't selfish or evil. My Lavellan believes in him still, too- one reason that I choose that dialogue line in the breakup. She can see that he isn't leaving because he doesn't care, but because (as he tells Cole) he believes he has no choice.