And I can just as easily say that Ren had fully embraced the dark side and was completely himself in that battle.
Then why was Rey depicted as being more in tune with the force?
It's like we have an equation. 2 + X = 5, and people really want for X = 2 when it's obvious it's 3. However, everything that we see afterwards implies that that's not the case. His acting, his desperation, and the fact that the OT has consistently shown that those who are in emotional turmoil are unable to utilize the force as effectively as those who are, and lo and behold, here we see Ren being beaten by Rey who is very attuned in the force.
If the situation only makes sense if Ren is in turmoil, what is the sense in acting like him not being in emotional turmoil is the correct interpretation?
The scene with the sun literally going out and all that's left on Ren's face is red, is symbolic of him giving himself up to the Darkside. The visual language in that scene demonstrates that he had indeed made up his mind, hence him tightening his grip on his lightsaber. The visual language of the film makes it clear that he's all in at that point.
Now I guess you can say that he was distracted since he didn't stop Chewie's uber blaster shot but considering he tracked and outraced Finn and Rey, his mental state wasn't that fucked up. Not to mention he really wanted to kill Finn at that point. Note how the film has Finn and Ren staring at each other after Han gets saber'd.
Finn has had extensive melee training so it's not particularly unbelievable that he could get a hit on Ren. Ren at the end did have orders to bring her to Snoke, which is what he was trying to do. Your explanation of Ren wanting validation is perfectly plausible but the bridge scene establishes that he's decided which road he's going to go down.
That's not how commitment works. For example, here's a person who adhere's to racist ideaology but still sees black people as human beings against his will, and this creates an internal conflict when he wants to adhere to KKK ideaology entirely but emotionally cannot. Still, he does what a racist is meant to do. He commits. And yet, even after he commits to his incredibly offensive act of racism and 'chooses his side', he is still conflicted.
Sorry, but this is just a fundamental misunderstanding of human behavior. The reason he was able to track and fight Finn and Rey is because distress isn't a delusional influence that make him see things that aren't there. It doesn't render him catatonic (though enough of it could, I guess). Ren could go out and murder his mother, Luke, and anybody that every loved him, but no matter how many actions he takes in the name of the dark side, if he still has doubts, they're not going to go away just because he forces himself to do what he does.