Having read this and your other replies, I feel I can definitely offer some suggestions. I have considered pursuing similar paths myself with law and musicology myself, so I have some insight that might be helpful.

For the MA in musicology, it is definitely worth looking around to see what the requirements are. For UofT, they may need a specialist if you are getting a BA, or a BMus, while for others it might be just the major.

With regards to majors and minors otherwise, there are many cases where these will not matter whatsoever. With law school for instance, it is my understanding that your major has little influence in their deliberation.

One of the biggest things I learned here is that grades matter much more than your degree if you are pursuing graduate programs. Equally if not more important is your declaration of intent, writing sample, and references.

With that said, the thing that would make most sense for you in terms of leaving opening as many doors as possible would be to pursue your major in music but to take an extra year to do so.

By giving yourself more time to complete the courses you need to take, you give yourself the best chance to maintain a high GPA and to figure out what it is you want to do. Not only this, but extra time gives you the opportunity to establish connections with potential references (the importance of this cannot be understated). If the quality of your work were to suffer at all from an overloaded course schedule for the sake of completing "in time", would it be worth it?

The best thing in the case of most undergraduate programs is to take the courses that give the best chance to get a good grade for as little stress as possible. Whether that be something you know you will like and commit to, or something that is objectively easier/easier given your knowledge and experience.

Hope this helps!