and the scene below with Clara holding her breath, and talking down the cyber-broom
That part was my favorite. Go Clara!

I am curious why she would be so resistant to regeneration
I'm wondering if maybe she's seen his other regenerations, but she's never actually seen him change? Because getting used to a different face and personality, semi-permanently, as well as dealing with his disoriented state seems like it would be more stressful than just running into other versions of him, helping them briefly, and then moving on.