Come on Lavellan. It's taken you 90 hours to start asking that question?
It's a bit like when they get to Skyhold and everyone is all just 'Gee whizz, it's sure good that Solas happens to know about this place. He sure has an astounding amount of knowledge on all things ancient and elven for a small town boy. I guess the Fade teaches him a lot'
I'm starting to think you are all much smarter than me... So, when I imagine I were my Lavellan... I meet that guy, looks just like your ordinary monk from next door, tells me he's travelled the Fade and seen the most curious things, studied a lot, went many different places... never really had many friends, too, thus having a lot of time for doing all that... discussed with demons and spirits, just sucking up every bit of knowledge he could possibly come across. Never heard of such people before, but then again, I have also never met a movie star or a Roman legionnaire and still know they exist(ed). Now, that guy has seen that orb somewhere, it is Elven, old, somehow magical. Much more he does not know though. It must have been what opened the breach, because what else could have had such power? And so on and so on, Skyhold, blah.
So, honestly, my first thought would not be "Gosh, he must be an Old God!" I might rather think "Kind of nerdy... damn, I wish I had such knowledge. Certainly impressive at every party or game show". My boyfriend can tell me a lot of things each day that I did not know, simply because I never cared to learn about them while he did. But it has never occurred to me that he might be an Old.... wait.
Seriously, was I really the only one who had been surprised by the ending after the first playthrough? Did you all see that coming?