This is tough. I hope that you father is doing well. Cancer can of course go multiple different ways, and sometimes it feels like luck which way it goes.
Physical therapy can be very helpful for a bad back, but again this can go different ways depending upon details.
How was your junior year of high school?
McGill has rolling admissions. At least way back when I was in high school, which was in a suburb of Montreal, there were a few (very few) other students in my high school who got into McGill based on strong junior and senior years of high school. They got their acceptances very late, after they had already put down a deposit somewhere else. I do not recall whether they stayed with where they had put down a deposit or switched to attend McGill. Most of us who were accepted to McGill at the time was based on our sophomore and junior years of high school, which meant that acceptances came earlier. I think that there were also a couple of students for whom McGill asked for their mid term grades senior year and then admitted them. I think that their business school is a reach with your overall GPA, but how this comes out might depend on your GPA for specific years.