The canticle describes an elf that freed his people from slavery and THEN joined Andraste to go after Tevinter. Have we seen any other lore that indicates elven slaves outside of Tevinter during that time?
I think an algamation might be that the old stories of Solas freeing the elves from the Evanuris might have been attributed to the elf that joined Andraste. Or there is another section of history with immense elven slaves we haven't heard about yet. And those elven slaves wanted to go back to their masters after they had been freed because it was easier.
I really like this theory. It fits in much more with how elven history has been told so far.
I'm a proponent of the theory that Mythal may have influenced Andraste - she 'nudged' history there much in the same way it was 'nudged' when she saved Alistair + the Warden's life. Apparently Andraste saw most of the chant's creation story in visions, which may be reflections of the veil being formed by Solas, or may be again, provided by Mythal. Mythal has shown that she's interested and involved - albeit, in a removed way - from the events of modern Thedas, and this depiction suits how we've seen her involvement so far. I find direct involvement a little more dubious - partially because of how messy Andraste's interpretation of real events is.
I would argue that Solas probably has very little to do with Shartan, simply because he does not value the lives of modern elves - what would motivate him to free the elven slaves when he sees them as shadows of what they once were? I also like the idea that significant elven heroes and villains exist outside the ancient pantheon 