I was left with the impression we got so little of Solas because too many questions/conversations would lead to too many suspicions, opportunities for more questions, and it may have resulted in fewer people being surprised by the stinger. There were so many times when my character would've wanted to call him out on his fishiness (much like you could with Blackwall.) In any case, I kind of hope that wasn't the case, because that's a bit of a cop out... so I'll carry on convincing myself it was a matter of time/budget. I respect PW and all he's done with his characters.
That may be true. It was funny, on my first play through I noticed how many codices kept popping up about the ancient elven gods, there are wolf statues EVERYWHERE (not just in the Exalted Plains or Emerald Graves either), there were several times Fen'harel was specifically mentioned, and how he tricked the gods, etc. So by the end I'm thinking... man this game is kinda elfy, as Sera would say. Then the temple of Mythal happened, and there's a specific convo with Morrigan about the Fen'harel statue, and Solas chimes in, and I'm just like... yea something's going on here with this Fen'harel guy.
Now, I knew by then BW was already planning a "Fen'harel wasn't really a bad guy" plot. This seemed obvious to me. So I shrugged and went along with it.
Then the stinger, and I was just like haahahahahaha why didn't I see this sooner? There are so many clues, but damn.
I think it helps you to understand how your own Inquisitor could be fooled for so long. Pretty meta really 
This is GORGEOUS but I'm also a little sad because I was going to have a tarot commission done...of exactly this. XD BUGGER.
Random thought on Solas:
He lies. A lot. Through his teeth. So when Lavellan asks...begs him to lie about his intentions, he won't. He will not lie about how he feels. The one thing he does not lie about, and it's his love for Lavellan.
ohgodithurts
It's weird but I personally don't think he lies, not really, anyway. I've tried to pay attention on subsequent playthroughs, and I don't think anything he says is an outright lie. He says he's an apostate, this is true. He says he lived in a small village, but spent most of his time dreaming in the Fade, I assume this is true based on Leliana's findings. Everything he knows and shares about ancient lore he says he learned from the Fade, which must be at least partially true (omitting that he lived through it isn't really a lie). When he tells you about the orb, he says it must be elven, and nothing he says is a lie, he just doesn't reveal that he knows *exactly* what it is, and that it's his.
I guess my point is, he's a master at the Game, and he never really lies. Which is why I personally think it makes his breakup scene all the more heart-wrenching, because even still, after everything, he refuses to sacrifice his ideals and lie to you, even though maybe that would be best in the long run.
I dunno, just the way I see it.