Here is my explanation on how Kallen is not OOC
It's my own belief that pre EP 22 Kallen did not believe in the Emperor Lelouch. As we see before 22 she wasn't falling for any of the stuff the BK had brought into. And the fact that she bothered to question Lelouch leads me to believe that she had still had 99% faith in Lelouch/Zerto and couldn't accept Emperor Lelouch at face value
Since we're told Kallen didn't realize she was in love until the kiss. We can assume that all she had in Lelouch at that point was faith. She had faith in his cunning, his smarts, his ideals, and faith in what she believed to be the real Lelouch. (This faith was instantly restored when we see Suzaku in EP 25)
In EP 22 she was looking for an explanation. So in reality she most likely instinctively believed that something like ZR was in effect. But she couldn't figure it out. She could not see the big picture and how the puzzles fit exactly. So she wanted there to be a ZR. She thought there
must be a ZR. But she had no proof there was a ZR, and there was enough circumstantial evidence (Euphie most likely) around to maybe keep her from being100% devoted to Lelouch.
Kallen saying she would go to hell with him means that if her faith was rewarded and Lelouch did have a plan. She would have gone along with it no matter what. Becoming a monster with him, Dying with him, killing for him, leaving her mother behind, etc etc. She would have been by his side
She had so much faith in him that she was ready to believe in a plan that she couldn't imagine or had any proof of.
So she went to to Lelouch for him to tell her what was really going on. Leouch would not explain. he stayed silent through all her questioning.
So Kallen had one last card to play. One last ace under her sleeve. HIS feelings, maybe just maybe, he felt as close to her as she did to him. So he couldn't possibly lie to her when faced with this truth. With the kiss she tried to strip away everything that was going on, the politics, the war, their personal dreams and ideals. She attempted to bring it down to an emotional level between just the two of them.
So she kissed him believing that the two were on the same wavelength that they shared a connection. That he couldn't possibly lie to her with everything on the table and with her at her most vulnerable point.
She wasn't saying "Please love me" She was saying "If you do love me please help me understand"
Lelouch gave her nothing. Which = "Nothing, any and all connection we had is now irrelevant and you no longer have a part in my life or future."
To Kallen she probably wrongly interpreted it as "This is all there is, Emperor Lelouch is The End Game"
The fact that she couldn't break the mask to her meant it wasn't a mask. It was the truth.
And we all know what happened next
EDIT: In addition as we can see from EP 25 that to a degree she had no problem with the end result of ZR (Not Lelouch's death but the peace afterwards). So we can assume ZR was not totally against her ideals and that she would have gone along with it.