So, was Solas a former evanuris that disagreed with their methods or was he a common citizen of Arlathan until he rose to fame as leader of the rebellion?
My impression was that the Evanuris were not physically distinct from the rest of the elves, but merely powerful members of their race (possibly of an aristocratic background anyway) who distinguished themselves in trying times, and thus became culturally distinct - viewed as gods. From having thousands of slaves at their disposal I imagine they have been able to hoard a large amount of power and became quickly impossible to challenge asides from by another Evanuris. The 'god' stuff is likely propaganda. All of the Evanuris fall into both categories.
As for Solas, my gut instinct - although it hasn't really been backed up yet - is to say no, he wasn't ever formally part of the pantheon.
A few small reasons: the symbol of the pantheon is an eight-sided star, which you see in multiple trespasser murals, likely symbolising all eight gods - Fen'Harel would be the ninth, and likely isn't included there. Secondly, if Fen'Harel was an evanuris that had 'turned', I think the gods would have treated his actions as to being more like a traditional war and given him more respect. That scenario probably wouldn't match the depiction in the propaganda codex we see in Trespasser, where Fen'Harel is a wild animal circled by the Evanuris, proclaiming 'Beware!' as if he literally is a dog. Also, I personally doubt he would have managed to gather as much support from the lower classes and been as dangerous if his dissent was generally perceived as a horizontal conflict - evanuris vs. evanuris, which usually means sacrificing their lives meaninglessly for the gods' gain - rather than a vertical conflict - the slaves vs. the gods, which threatens to knock down the structure of society.
So I don't think Solas was ever formally inducted into the pantheon. Fen'Harel was obviously recognised as a worthy threat and well known enough that in the shattered library, ordinary citizens were able to specify him as the cause of the veil being formed.