It was kind of you to switch with your neighbor, for her sake and so that there would be less stress on the "wilder mouse". As I was reading about your experience the phrase First, do no harm was running through my mind. Even though I realize that there isn't a way for that to be possible yet, it is heartening to know that there is great emphasis put into making each step as painless as possible for the mice (subjects). I realize that these ethical procedures aren't anything new, but your personal experience and outlook was still comforting to read.

But I think there is one thing they’re teaching that might not be so obvious: compassion for the animal.

A fantastic point. Scientists, researchers, doctors, veterinarians, etc have the difficult task of having to be humane and clinical. As you wrote, sometimes things go wrong and choices need to be made fast. They are necessary but, not enviable. My heart goes out to you for having to perform the kill. That you were able to do it swiftly for the mouse's sake has doubled my respect.