Mickey does conduct the instruments so it is his philharmonica really, so it kinda makes sense. Besides it just rolls off the tongue better. But you are right, Donald is the "star" of the show as it were, getting the most airtime.
I didn't know what to expect of the show, hearing from a couple old CM friends that it was another cheap Disney 3D movie, but I was really pleasantly surprised. The image that sticks in my mind the most is when Ariel reaches out of the screen. I'm not that sentimental, but that one got me big time. Of course it's a touching moment, but not only that, but from a 3D perspective, again which is the kind of art I do, I was amazed at how it looked! It had to be one of the most incredible moments of 3D I'd ever seen. So crisp, so artistic.
My only fault was not in the film itself, which I found delightful, but the shortness of it. I wish they'd put more scenes in and made it longer. Maybe they will someday. After all, there is a lot more Disney films to get into. I will definitely go back and see it again the next time business takes me to Orlando. And hopefully soon take the family.