Most left handed people I know (me included), use the mouse with the right hand. Might be a bubble though, but I never thought to get a left handed mouse, or left handed scissors or most left handed things. There's a few things that really don't work (I play my drumkit with hands reversed), but most things are fine.
I tried using a mouse in my left hand in left handed mode back in the day. Eventually, I got tired of changing the settings on shared computers at home/school, and I realized it wasn't any more or less hard to use the mouse with my right hand. Today, I use a right handed vertical mouse. Interestingly, I play guitar right handed, despite being inspired to play by Kurt Cobain. My girlfriend got me left handed scissors as a joke, but man, they actually feel better.
My sister used to use a neutral mouse, with her left hand, on the left side of the keyboard but in "normal" or "right handed mode". Now she has a laptop with a big touch pad in the middle.
I do what your sister used to do. (Left hand mouse use, buttons left at default).
Even game that way. Started using it that way as a kid, by the time I learned it was possible to switch the buttons that seemed less natural than leaving it as default.
Also made it much easier to use shared computers in school labs and the like.
I am lefty and occasionally use the mouse left-sided when I have right arm injuries. What I don't do is swap the buttons. That way, nothing has to change, I can switch to the preferred side at any time, and coworkers aren't stymied when they need to drive the computer.
Keyboard shortcuts aren't as convenient with the mouse on the left. The most useful ones are all biased to the left of the keyboard. Left Ctrl is also easier to hit reliably.
In school for fun when it was boring I many times would do writing exercises with the right hand (like write a line of A's, a line of B's, etc) and after a couple of weeks I got the right hand up to maybe 80% speed and accuracy of the left, but I realized I needed to do constant training to keep it as good as the left (admittedly the left had daily training of many hours due to having to take notes, etc). But it does sound plausible!
I would have been left handed when an early childhood injury caused me to switch to right. A few years ago I thought about that and tried re-learning some skills and tasks with the left hand.
It's specific to each task but I can normally get the left handed version as good or better than the right. I am willing to bet most people could do this, you just have to spend a bit of time awkwardly re-learning.
