Didn't the girl's aggression almost kill everyone?
Still… no punishment.
I swear that girl verbally insulted you and you almost killed everyone, and still no punishment…
I swear… bru
He prevented her punishment because he could see that that would lead to his death. Maybe if she gets expelled she decides to just murder him instead of trying to humiliate him first or something like that. He's looking out for himself first and foremost.
This is actually a neat twist that I haven't seen often. The villain has already done a number of villanous things, and he now has to sort things out in a way that lets him survive. He can fix his faking his way to a professor's position by actually becoming stronger, which all the traits he gave himself will help with. But finding forgiveness from someone whose father you drove to suicide and took everything from? That's going to require a fair amount of gaslighting to smooth over.
I can imagine him trying to explain that it was all a misunderstanding, that he didn't steal her father's research, and so on. Express remorse over how things went, while becoming her lifeline. She would have been very young at the time, so maybe he can trick her, and if she depends on him she will rationalize things on her own. She's clearly starving for attention.
The fiance though is a tougher nut to crack. Maybe he could deflect her ire to their family heads, like he wants to break the engagement too but can't. After all, the real conflict there is that she is tied to him, not the actual crimes. As in, she could tolerate it if he came clean. A solution there might be to just become stronger and earn his position, although with her being a knight it might not be enough to "fake it till you make it".