If some followers of Mythal survived in her temple , (Abelas) there's no reason , followers of the Dread Wolf couldn't do the same.
We know Solas was in the fade , and we also know he wanted to do something (we just don't know what exactly).
I'm pretty sure Felassan's boss was Solas all along , just Solas was still in uthenera and communicating through the fade .
If I remember correctly Felassan doesn't worship the elven gods , he doesn't bow when he sees statues etc ...it fits with Solas not seeing his peers as "Gods" .
Also Felassan knows all the stories about Fen Harel , and his name comes from a story about Fen Harel ...Abelas picked his name when the temple of Mythal fell (you can find a text in the temple of Mythal about that)
So again if the ancient used to change their name to fit their "role" , Felassan is linked to the Dread Wolf.And it's not just a coincidence.