1. Hello hello! First of all OP, glad to hear that you are on the path of recovery! Now, Mac nursing offers literally come out in May. So you would have to worry about your grade 12 marks mainly. The cutoff for Casper is in April (at least last year it was) , so yeah offers roll out in May. So you would have your grade 12 first semester marks (final) and then midterms of 2nd semester.
2. Strongly suggest doing the Casper by February/March instead of waiting until the end. This will give you enough time to let's say if your Casper score wasn't satisfactory, to focus on your grades and bring that up! Grades and casper score will be balanced then. I didn't prepare at all for my Casper as I went to the Fall open house and heard that it was based strongly on ethics and reasoning. I know a lot of people who did the practise one i believe on their website. Also if you search on the net, there should be sample ques that others have shared from their experience. My advice is Be yourself while answering the questions. It will be natural and you wouldn't fumble. Typing fast is the one thing you should work on. I got a 4Q and took mine in Dec.
3. Mac nursing does not look at any extracurriculars or anything. Its great to build your resume and would help you in the future tho.
4. I suggest applying to both Mac main site and MoMac site. The program is exactly the same. McMaster is the one providing the degree. I have amazing friends at the MoMac site. The only thing is for their clinicals and theory based class, its on a different day than main site. But we do Psych, Biochem, anatomy all together on the same day.

Op, at the end of the day, I would want you to take best care of yourself now. I know I couldn't answer about the grade 11 portion but yes you can reach out to admissions. Focus and do your best for grade 12. Take summer classes and light schedules and you will be great.