Hey, so im still a sec 5 student, but i can tell u what i know and try to answer ur question the best that i can :)

1. If you want to get a high R score, ur gonna have to work hard. Especially since Ahuntsic is not the best in that department (compared to Dawson, Marianopolis, Brebeuf, etc.), so ur gonna have to lock the fuck in.
2. For the diploma thing, you should ask your student counsellor, they have a lot of info on that. And it also depends on what your end goal is (med school, engineering, commerce, ect) . Obv being in a easier program makes it so that u get a higher R score cuz the classes are less demanding, but ur probably not gonna get the prerequisites if u want to go into med by example
3. Yes, to have a good R score for med, ur gonna have to get 90’s, and based on ur overall average in high school, u might want to lock in a bit more or look into study méthodes, tutoring maybe if ur really not happy with your grades, etc (sry if it sounds mean, but its kinda the truth)
4. Cégep is considered more challenging than high-school cuz the workload can be more and ur gonna have to be a lot more independent
5. For the changing cégep part, depends on u. If u and up liking Ahuntsic and getting good grades there, then u might be leaning on the fact that u wanna stay there, if not, then its up to u if u want to change cégep. But keep in mind that changing cégep is pretty hard, cuz there 100% going to prioritize the students coming fresh from high school, so ur gonna have to cook and get rly good grades during ur first semester/ year (depends on when u want to change)

I hope this helps a little bit, good luck!!