I think that Felassan was to Solas what Abelas was to Mythal, except perhaps more so. More like a... steward or something (based off of Felassan's little "do you even KNOW who I am?" talk with Michel).
I think he was working for Solas the whole time, but then someone else killed him. Don't get me wrong, I totally think it plausible that Solas would've killed him if Felassan betrayed him, and I think Felassan knew that too, but the way he died in the book doesn't sound like Solas' stye.
First of all, I'm assuming that Felassan was not working for one of the trapped gods. The whispers in red lyrium in the fade are... a little crazy. They do not sound like they come from anyone that has a terribly strong grasp on sanity. Which to me implies that the creators wouldn't really be functional enough to have an errand boy on the outside dipping his hand into the subtleties of Orlesian politics. Also, if it was someone other than Fen'Harel that Felassan was working for, I doubt they would've much cared about the plight of the elves of Orlais. Seems to me that they'd be more focussed on trying to get out of their prison.
WE ARE HERE
WE HAVE WAITED
WE HAVE SLEPT
WE ARE SUNDERED
WE ARE CRIPPLED
WE ARE POLLUTED
WE ENDURE
WE WAIT
WE HAVE FOUND THE DREAMS AGAIN
WE WILL AWAKEN
The way Felassan talks to the person in the fade is not the way that I'd expect someone to address a seriously pissed off, corrupted powerful being.
Assuming then, that Felassan was working for Fen'Harel, I made the leap that Solas didn't kill him purely because it doesn't feel like his style, and because it seems a little too obvious for a life-ruiner like Patrick Weekes.
1) Fen'Harel, in all the legends, is a man with a plan. He does not seem like someone that would willingly toss away a tool if they could still be of use, as has been said before. His banter with Sera shows that in spades.
Solas: I heard about your organization, Sera. I am impressed.
Sera: Is this a trick?
Solas: Hardly. But it is an opportunity. You have already divided your group's membership. That is wise. No one cell can betray all your secrets. The next step is to establish a rhythm. When your enemies pursue, you vanish. When they become complacent, you harass them. When they are weak, you strike in earnest.
Sera: Where d'you get all this, then?
Solas: Do you wish to be unnerved by another tale of my explorations of the Fade? Or do you wish to learn something?
Sera: I don't know. Neither?
───────
Solas: Once you have the aristocracy weakened, Sera, you will have to redirect your lieutenants.
Sera: Oh, this again. All right, what am I doing?
Solas: Some of your forces, valuable until now, have no interests beyond creating disruption. Chaos for its own sake. They must be repositioned where they can do no harm, or removed if necessary. You replace them with organizers willing to build a new system and carry out the ugly work that must be done.
Sera: What? Why? What ugly work?
Solas: That is up to you. Do you wish to disrupt the nobility, secure a title? Or change the political structure entirely?
Sera: None of it! I don't want any of that!
2) Solas would've let Felassan finish his sentence. He would have probably raged at him a bit too, not killed him in cold blood from behind if he was going to kill him. In All New, Faded for Her, Solas straight up murders some dudes if you let him, and they know FULL well why they're being killed, or at least, he doesn't hold back on letting it all out at them beforehand.
3) Solas' greatest fear is being alone. Assuming again that Felassan worked for Solas, Solas having someone that remembered everything just the same as he did, someone who wanted the same things as him, someone that he likely fought alongside back in the day? He would not give that up so casually.
Also, other random thoughts that are far more conjecture-y that I've been dying to discuss with people (I'm not necessarily sold on this, but it feels just as likely as anything else at this point):
- What if Shartan wasn't Solas, but Felassan. I bring this up because of the symmetry between Briala/Felassan and Andraste/Shartan. Two underdogs trying to take down the system.
This is based on gut feeling more than anything, but why did Solas wake up when he did right before Inquisition? Things are decidedly not worse now than they were when Tevinter conquered most of Thedas and enslaved everyone or when the Dales or Arlathan were trashed. I posit that Solas woke up as soon as he possibly could, and that's why the timing worked out like it did. If he had woken up and become Shartan, why would he not be trying to unlock his foci back then? To free the other creators then? The world had already gone to ****, why would he not have tried to start whatever he's put into motion in Inquisition back then?
Bonus Conspiracy theory: What if Felassan was ordered to not only help the rebellion, but to also reclaim Fen'Harel's foci? Just like he not only helps Briala, but is doing something else on the side.
Ok, so, Dorian talks about the orbs with Solas in banter:
Dorian: That orb Corypheus carries... are you certain it's of elven origin, Solas?
Solas: I believe so. Why do you ask?
Dorian: There are paintings in the Magisterium's archives of men holding similar orbs.
Dorian: They were depictions of a time long before the magisters. The ancient Dreamers, perhaps.
Dorian: The texts called those orbs "somnaborium"--"vessels of dreams." Could they be the same thing?
Solas: Perhaps. The humans of ancient times took much from the elves.
Dorian: And Corypheus isn't far removed from the time. Hmm.
What if part of the goal of Andraste's rebellion was to get into Minrathous and reclaim the orb? What if he took up the name "Slow Arrow", not just as an homage to Fen'Harel, but because that's literally the role he's been playing while Fen'Harel slept and regathered his strength? Putting things into motion way back when so that when Fen'Harel could finally come back, everything would fall into place. The world would be primed for what was coming. "When your enemies pursue, you vanish. When they become complacent, you harass them. When they are weak, you strike in earnest."