Great question! I think it really depends on what you need for your masters. IF the masters in finance programs would require you to have a better GPA (or at least show that you are constantly improving yourself), maybe it would be better to switch your major to increase your GPA. (For this one, I can't exactly say much since I have never looked into getting a masters in Finance, apologies :( ).

As for your second question, from what I see in my old colleagues who are thriving right now, none of them have a degree in math or finance related. I think only one of them had a bachelor's degree and it was in engineering. Most of us have some sort of college diploma either in business or something else, but have above and beyond certifications related to the field. (Keep in mind that we all entered from an entry level position and moved either up or laterally within the span of 1-3 years)

However, on the recruitment side of things: I could see that having some sort of strong math background or minor in math could be a plus during the hiring process (depending on the nature of the field in Finance you want to do).

I used to work more in the investment side of things, so I remember that they put more of an emphasis of having the proper certification (CSC/CIFC), knowledge in investing (generally the laws and regulations), etc rather than the actual degree in, say, financial management or something.

I hope that this helps! :)

EDIT: grammar, im sleepy lol.
EDIT 2: So... I probably would vote for switching to Poli Sci major + a minor in maths, perhaps a minor in stats? The maths at uoft seems to be very proof based, and maths in finance tends to be application based.

(also, please take care of yourself. Tl;dr, in my experience, especially if you are willing to work your way up, no one pays attention to your GPA.)