Sephiroth is a fascinating villain because he is one who ultimately fails due to his inability to understand the complexities of the human heart (or I guess you could say he underestimates the power of love, lmao.) He spends almost the entirety of Rebirth trying to drive a wedge between Cloud and Tifa, but ironically, the one time he seems to succeed is the one time he hadn't set out to do so.
He seems to know Cloud quite well in that he understands that Cloud's greatest weakness is his love for Tifa and his desire to protect her. That's why he's first able to control him in Gongaga, when Cloud in his Mako poisoned state rushes up to help her. But what Sephiroth doesn't understand and underestimates is Tifa's love for Cloud. This is a Tifa who doesn't love Cloud because he's a hero or her protector, but for who he truly is.
Tifa sees Cloud on a murderous rampage, she sees him trying to kill her, and instead of becoming terrified, instead of distancing herself from him, she commits to protecting him and stays by his side closer than ever. Even in the Temple of the Ancients, when Cloud is again on a rampage, and lashing out, she is constantly reaching out to him, putting her body on the line even when he pushes her away. Because she knows that this isn't Cloud. This is Cloud acting under Sephiroth's influence. The real Cloud is still there, even as it becomes harder and harder to reach him.
At the Forgotten Capital, I don't think Sephiroth is thinking about Tifa at all. His primary goal is to kill Aerith/stop her from calling Holy. His secondary goal is to fuck with Cloud and taunt him about his weakness/inability to save anyone like he did during his ToTA trial. (Also, Sephiroth is not the most subtle villain in the world, lol. If Tifa was part of his plan here, he would let us know.)
Cloud's delusion that he saved Aerith/that Aerith is still alive is one of his own making, and that's why it affects Tifa so much. The violence she can explain away, that's clearly Sephiroth, but this, this absence of grief, this lack of kindness, this she cannot. He is no longer the boy she grew up with. He is a stranger.
Cloud's mental state at the end of the game may come as a shock to the player, but I don't think that's the case for the rest of the party. Barret and Red have been talking about how weird he's acting as early as Ch. 3 and again in Ch. 11. Cloud being delusional is just par for the course for them. But with Tifa, it's different. She's spent the entire game trying to pull him back from the abyss, but now she can't speak to him. She can hardly even stand to look at him.
This wasn't part of Sephiroth's plan. Cloud did all the work for him himself. Ironically, it's when Sephiroth tries to press the issue again at the Northern Crater that it completely backfires.
But this also totally tracks with Cloud and Tifa's characters. The greatest obstacle between them has never been an external actor like Sephiroth, it's always been Cloud's internal conflict: his weakness/inferiority complex that made him so susceptible to the Jenova cells' influence, that allows him to create his own illusory world, all because he couldn't believe that Tifa could love him for who he truly is. It's only once they resolve this that all the other pieces fall into place.
