I get what you mean! But, it doesn't change the fact that there are two entities, even if the Solas we see now is made up of both and wouldn't exist without either. Flemeth may say that Mythal and her are as close as a body and a heart- but she's still able to remove the part of herself that is only Mythal and give it to Fen'Harel.
Can we really say what the original Flemeth would have wanted? She traded her life and her soul away for revenge. Possibly the souls of her own daughters down through the ages.
There's always going to be a big difference between Solas just being Fen'Harel and being a combination between the two. I prefer the simpler answer, but I wouldn't be that bothered if its the latter. But I do not see the two as being analogous. They are two very different story situations.
I see the point, I just don't see why any of that matters when the man you fell in love with is this guy currently.
I'm a much much different person than I was five years ago, and if people saw me then they might not like me as much or at all. I certainly get the objections, tussling with the idea of it and trying to figure out who is Solas. The concept makes him feel less real.
But then, he is a god. Did you all really expect a relationship with such a being to be anything less than complicated? 
Of course I know you all know that. Rhetorical. Lol, it also wasn't exactly your choice, but it's something to think about anyway. Even if Solas was always Fen'harel, the two personas, him then and him now, will be so different that they may as well be two different people. And I can imagine if he is a villain, that he'll be in conflict with that regularly.
Like Anders style separate him from Justice I think.
And I'm not saying Fen'Harel's bad and Solas's good. If anything, if they actually were once separate and I was forced to choose, I'd probably pick Fen'Harel because he's much more interesting. But he still wouldn't be the Solas I got to know.
Though while writing this, I think even if Solas and Fen'Harel were separate once, I don't think it's possible to separate them again. Take Anders. Even when you're on his rival path, and it looks like Anders wants to get rid of Justice, they still can't separate. It might be different with a god's soul, but if both Solas and Fen'Harel are willing parties to their binding, I don't think there's any way to force them apart if they don't want to.
I don't think they can be either. Not without someone dying, like in the case of the archdemon, and even then the souls here are what is linked. It's a lot different than what we've seen before, if true.