I just graduated from RMC. I got a BEng degree. I think that it is a fantastic idea to actually go to RMC and talk to the Cadets there. Although not the first years, you can, but try to talk to fourth years, and not just one. I really wished I had. Some people love it there, and others hate it. I was one of the ones that hated it.
I have had many conversations with my peers over the recruiting centers. A lot of them felt as though they'd been lied to or mislead. Just like everything else, there are some good recruiters who really know their stuff, and others that just don't. None the less they are the ones that know the system. So for all your questions about chances based on various factors, they are the only ones who would have any chance of knowing.
All I want you to take away from this is start asking the right questions. If you talk to Cadets ask questions about lifestyle, about their courses, find someone who has the trade that you are interested in. The training is constantly changing, and they'd be the most up to date on that topic. Again try to find the older Cadets. Also what no-one seems to have told you, is that there is the ROTP civilian university program. It is in no way the lesser option. There are many benefits to it that RMC does not have. Including a much wider academic choice. I also think that more independence is gained through going to civy U, provided you won't still live with your parents. People go from the influence of their parents to the very oppressive influence of RMC. Without ever having developed on their own.
From what I've experienced marks are a large part of who gets accepted. There are a lot of very intelligent people at RMC, book smart that is. Socially smart? That's a little different. I also have friends that got in with comparatively low high school marks (I personally was about 85 or 86%). Some were recruited for sports, others just applied in the right year. Some may have wowed the interviewer, who knows? Remember there is also a waiting list!
The point is, before you accept find out as much as you can about it. If you really want to be an officer there is more than just RMC. Determine if you really want to go to RMC (there is also the first year trial to determine if it is right for you). Don’t dismiss the NCM world, there are some really exciting opportunities only open to them. And, about being a leader, most of the good leadership examples I have experienced have been senior NCMs/NCOs. So find out what you want and apply, and stop wondering about what the chances are, because it REALLY DOES'NT MATTER! Best of luck!