1.) Perhaps. All the gods seemed a bit jerkish, but to tell the truth, and after the Fen'Harel business, I'm kinda reserving my overall judgment on them until we know more. And as much as I love Solas, it's occurred to me that his own perspective might be colored by his own biases. I'm not saying the elven gods were roses and sunshine (far from it), but I'm a lot more wary about deciding who they are without knowing all the details.
2.) We don't actually know when Solas woke up. We assume things, but we don't know. And Felassan was contacting whoever he was working for through the Fade. Solas could have still been in uthenera while orchestrating these things.
3.) Solas is trying to free whatever (elven gods or ancient elves) from behind an eluvian. So gaining full access to the network would be in his interests. Solas dislikes the Dalish and he dislikes having to directly lie, so he may have found it easier to have Felassan work with them. "Felassan" also happens to mean "Slow Arrow", which is the weapon used by Fen'Harel in the story he tells. I've also wondered if they knew that some sort of sacrifice would be needed in the bargain for the keystone... It may not have been an accident that the most jerkish Dalish clan in Thedas was targeted.
4.)Generally, the evidence points toward Felassan's killer being Fen'Harel. There's no evidence for Falon'Din or anyone else, so I can't really believe those theories until we have more to support them.
Do we actually have evidence that he's trying to free the Creators?
I mean, we get dialogue with Cole that basically points to him trapping them beyond eluvians, and feeling guilty but justified about it.
But I just don't believe that Solas thinks the solution is to release the other gods. Especially if they were blighted. I just don't see Solas being Felassan's killer making any sense.
(That is not to say I don't think Solas is that ruthless, it's just the wrong kind of ruthless for him.
) And Felassan refers to his 'clan' as being dangerous, and doesn't seem fond of them. Solas makes constant references to having been alone for a long time. My two cents is that Felassan was part of Falon'Din/Dirthamen's Sentinels, trying to break out their god on his orders.
We have the whispers written in red lyrium which point to some kind of dreaming force having woken up again, and are trying to get out.
We have Felassan's talk of his clan.
We have Dirthamen's presence in Tyrrda Bright-Axe as a power-mongering Creator apparently opposed to Mythal and willing to attack Arlathan itself.
We have the statues of Dirthamen in the Fade where he's stuck with a sword and bleeding.
*shrug* I feel like Inquisition and The Masked Empire have been setting us up for Dirthamen/Falon'Din as an antagonist. And if the Elf God=Old God theories hold any weight, the two Archdemons left are the twin-souled Creators.