Although I'm not enrolled, I do have limited experience and can answer a couple of your questions.
From what I've been told, you should apply in grade 12. I'd suggest getting it all together and sent in by the end of September, I've heard of a few people who have had papers lost, and I personally am wrapping up medical issues. So the more time you have to sort everything out the better.
Chances of getting in will change from year to year, the higher your average is, the better your odds, and I think Air Cadets will help, but I'm not a recruiter, so who knows. Also, get some leadership experience, it will help out a lot.
I have a cousin who goes to RMC, and a friend who goes to my current university who is in the military. What I've gathered is Civi U is great to help social interactions, not saying RMC folks aren't social, and you get more real world experience. Not only that lack of inspections, and more control over your schedule can reduce stress. My cousin told me he'd sleep on his bed with his ranger blanket to make his mornings easier.
The advantage to RMC is a good education, and I've heard their engineering department is where they excel, but that is second hand information. RMC also helps develop you as a military personal, you will be used to the high stress demand that will go along with your career choice. Also you have the chance to make a lot of friends who you might be stationed with.
Like I said before, this information comes from my current application process and some friends who are currently in ROTP.