I may be remembering it all wrong but I got the impression Mythal was always a spirit of justice, rather than the protector. Now the Dalish teach that Elganan (spelling could be wrong) was meant to be god of vengeance whom Mythal was able to subdue and calm. So I wondered if in fact they were two aspects of the same god/sprit.
Mythal wants revenge for her murder. She did before she merged with Flemeth but was not powerful enough in spirit form. She was drawn to Flemeth because she also wanted vengeance against the murderer and deceiver of her lover. The spirit Mythal gave Flemeth her long life and over the course of this her power grew, fuelled by Mythal. That's my take on it any way.
Whatever occurred in the past, Mythal does not hold a grudge against Solas/Fen Harel. If the scholars are right and he was more a god of rebellion, may be it was the murder of Mythal that ultimately focussed his resolve. He goes to her because he does not have the strength to do what he wants on his on. A joining of these two entities gives greater power but at the loss of individuality. I think Anders/Justice could have been a foreshadowing this relationship. Justice gave Anders the focus and drive to work for a rebellion against the status quo but Anders' passionate hatred of the system meant that Justice became Vengeance. So in absorbing Mythal and her desire for revenge, Solas has a greater potential for dark deeds.
Do you suppose that there is an orb somewhere that belongs to Mythal? I just been discussing the orb again with Solas and he says how it is a focus of power and that there were others, each dedicated to a god. I'm wondering if they act something like the one ring in that the god stores something of their power within it and so long as it exists the god cannot be truly killed. So Mythal can keep coming back so long as her orb is safe. If this theory holds true then Solas lost his immortality with the orb. No doubt we'll find out eventually.