It all comes down to answering a few questions:
-What was Mythal's betrayal?
-Who is Fen'Harel, what is his relationship with Mythal and what did he do?
-Why are dragons Old Gods? How do they communicate with people telepathically?
-What is the taint? Why does it threaten the Elven Gods?
And of course, What are the Elven Gods? Merely High Spirits?
MMm I think from what I saw in the temple of Mythal , this is my crazy theory about what happened:
There were some elves seen as "Gods" , I have no idea what their true nature was , but they were way better mages than anything possible in current Thedas.
Those beings were able to turn into high dragons , and it was seen as a divine form .
Now you weren't born a god , you were made one.
That's what the story of Ghilan' nain is about.She was an elf , it seems a follower of Andruil , and started to create monsters left and right.Andruil liked her well enough and decided to give Ghilan'nain a place among the Gods.
Now the elven gods were cruel , Falon Din was a tyrant , Andruil liked to have sacrifice made in her name , and it seems Elgar'nan fury was pretty terrible.
Mythal was keeping thing in order though , she saved Andruil from madness and corruption , fought Falon 'Din , and also was busy judging the elves who were tired of Elgar'nan justice (I imagine his view of justice was burning places down).
At this point I assume Mythal was one of the most beloved Gods.
Then Ghilan'nain went to an elf (someone following Dirthamen , BFF of Falon Din) and told him he should turn into a dragon.I suppose she told him if she was made a God , it could happen to him too afterall.
Thing is shapeshifting into a dragon was seen as Divine and only the Gods could do it , so the dude basically made a big etiquette mistake.
He asked Mythal to protect him .
Mythal decided to not deal with that , and let Elgar'nan judge the man .
Now I suppose at this point Mythal is in a terrible position , she was seen as harsh but fair ...but what could she possibly do?
If she forgive the man , now her people might think Godhood is for everyone , and people would start turning into dragons left and right.
If she punish him , she's unfair , the dude was basically tricked by another God.
So she just washed her hand and let mister no self control ElGar'nan bring justice.And Elgar'nan probably killed a lot of people .
It seems the people instead of being scared into submission , started to get really pissed off .
So a rebellion begins and some decide to kill the Gods , Andruil gets involved in this ...I believe she killed Mythal .Maybe she wanted Mythal position , maybe she was unhappy Mythal fought her and won in the past , maybe Andruil got corrupted deeply by the "Void" and was furious Mythal erased her knowkedge of the place.(think Corynoob who got tainted by the Golden city and still wants to go back there anyway)
Anyway whatever happened once Mythal is killed , chaos for everyone!She was the one ruling the others gods and keeping everything in check.
It's possible Fen Harel was at first involved in the rebellion , but probably decided to backpedal once Mythal was slain.(afterall Solas says he enjoy court intrigue , despise all the slavery , and that he made the mistake to trust his friends in the past , so I'm thinking he got fooled at some point )
Another possibility is ...there's not much talk about Elgar'nan involvement .It's strange.
Also Mythal was draw to Flemeth who got married to a lord purely for pragmatism , then her husband betrayed her and killed the one she used to love.
So errr...were Mythal and Fen Harel involved at one point?Or at least got close enough , Elgar'nan got jealous , bought himself a brain , and made a plan to overthrow his wife from power manipulating their "children"?