(...)I prefer to think of it as the dalish being wrong: again.
I don't think they are actually wrong in this one case - they just have no information. I might be wrong, but I think they don't really know whether the tattoos actually existed in Arlathan or are an invention that only appeared in Dales and then were popularized among the clans as a sort of declaration tattooed on their faces. They are wrong in many thing, but one can't be wrong about something he has no real opinion on 
In this context it's actually Solas that's wrong in this case since Dalish tattoo their faces for their modern reasons rather than "because ancient elves did that", which makes ancient meaning of similar tattoos completely irrelevant.
Although, as for Dalish being slaves - we have reasons to suspect that most modern elves (if not all of them) are, indeed, descendants of Arlathan lower classes and slaves. First of all, if Arlathan was anything like any other empire ever, the slaves/commoners greatly outnumbered the nobility. And then
On the other hand, we could look at whole elven history and ask ourselves - to what extent is it simplified?
We have Brecilen ruins, out of Tevinter reach until right before Andraste's Exalted March. Ruins that seem to prove that elves and humans lived alongside each other. Was this part of Elvhenan? Or maybe they were some outcasts on the fringes of elven civilization? Or perhaps they were the remnants that Tevinter didn't actually get to?
In elven history we had a pretty unified vision of the race:
1. Elvhenan gets unified
2. Elvhenan falls and surviving elves become Tevinter slaves
3. Tevinter falls to Andraste, some elves remain in Tevinter but most move to Dales
4. Dales fall, most elves get sent to Alienages across the human lands and forcibly converted, the rest become nomads
This is pretty clean, but that seems fishy. Basically:
1. Do we really assume they lived happily as one great empire? That seems like a bit of a simplification, history - including Thedas invented history - implies that unification on such a grand scale never is complete (though we also have dwarven empire)... Basically, this raises an eyebrow, but I could take that. Then things get more complicated, however:
2. Tevinter never in its entire history managed to subjugate all of Thedas. At the height of their expansion they sorta controlled Ferelden Valley and they had no control over Chasind lands. We encounter elven ruins in part of the world that got under Tevinter control much, much later or never. There must've been some elves that remained free - I wouldn't be surprised if they were actually the reason why any elven lore actually got resurrected in Dales at all. After all, a couple hundred years of slavery is a lot, especially with elven culture explicitly banned. Also, even in lands controlled by Tevinter - one would think that perhaps some proto-dalish refugees hiding in forests could form, the land was likely much more wild then. And it' s plenty wild.
3. Elves lived (as slaves, generally speaking) through the whole Tevinter empire. And then they are made free and some distant homeland is created. They pack up and they go to seek theiir happiness in new lands... But why? Why would all of them want to move? We're told that some of the elves turned back to Tevinter or stayed there, but if the journey proved too hard, why would they want to go back? And the "don't feel the urge to go there" kind - why would it be only in Tevinter? We're talking about people who recently got freed and are viewed as allies of the martyr-prophet whose cult just gains momentum. What's more - we're talking about people who, some of them at least, actually fought for said prophet. It seems only natural that many of them actually embraced the new faith and didn't necessarily feel the urge to go and create the new elven homeland. It seems only natural that there actually existed a big population of proto city elves long before the Dales fell. On a side note - I must say I feel really bad for those elves as they are the ones that got screwed the most by history, when Chantry decided that the elves are evil demon-worshipers, struck Shartan out of the Chant and proceeded to stuff former Dalish citizens into alienages across the world, those elves that were actually living in human lands and likely worshipping the Maker already got royally f*cked as a ricochet. And it's not really all that impossible that they were the first to end up corralled in alienages and consisted a majority of their populations initially. After all, what's easier, find any elves you already have, stuff them behind some wall and say "oh, look, we have an Alienage" or actually go transporting elves fron Dales, half a continent away?
4. I already touched on this, but from what we know it seems that a lot of elves live in Dales now, Halamshiral is a mostly elven city too. Also, while we're supposedly told that Dales was all about worship of their ancient gods, I frankly find it unlikely that there were no worshippers of the Maker that got either killed in retaliation by their neighbors (when you have an Exalted March inbound you're not likely to look favorably on those that worship the same god the invaders do) or later disappeared when the general population was more or less converted. However, my thoughts are that, in general, it is overestimated how many elves were actually shipped away from the Dales. Also, we know that elves serve as farmhands - this would imply that elves actually live in the country too, not only in Alienages, in cities. I personally believe that while we hear of the "city elves" the majority should really be called "village elves" or something along those lines. After all, there seems to be much more work for additional hands in the country rather than in the city...
Of course, some if not most or all the things I'm writing here are a result of a dangerous attempt at bringing common sense to a fantasy setting. Something that, I believe, I've read called along the lines of intellectual self-pleasing. Still, since I like creating worlds and attempts at making sense out of a setting (a habit I picked up due to my interest in p&p RPG)... I decided I could share my thoughts before the thread gets closed down due to flame