I think your last question is the best one to answer first.
Regardless of what anyone on here tells you, it's still up to the recruiters, so you should definitely talk to them - even if you're not planning to apply to the Forces this year. Recruiters are there to help you, and are capable of both answering all questions you have and directing you on how to make yourself more appealing to the Forces. If your local recruitment center is far from you, the forces.ca website has an online chat with a recruiter function. I found my face to face with a recruiter much more informative than the online chat, but maybe that was just my personal experience. You can also give them a call. They'd be happy to talk to you, no matter how far off the mark you think you are when it comes to the ROTP standard.
As for your marks, I was repeatedly told during the recruiting process that a large portion of the application is weighed on marks. The aptitude tests are also taken into account, so if you do extremely well on them that might balance out some of your marks.
You should volunteer wherever you feel is right; during the recruiting process you might be asked why you volunteer there, and I don't believe that "because it's a way into the Forces" is a very good answer. With volunteering (and this is my personal opinion) I think they're looking for personality characteristics like your ability to believe in a cause and put time and effort into it. For the same reason, hobbies are also a bonus, especially if they're related to your future working field. Let's say you're a computer engineer... if you like to create websites in your spare time, that's a great hobby to list because it shows you work on your own to develop skills you'll need in the future.
Same with the sport; choose something you like and stick with it. If you like martial arts then you should do martial arts - I personally started karate when I was 17, and nearly half the beginners in my dojo are adults. Most dojos hold child and adult classes separately, and have a sub separation for belt level; it's not nearly as awkward to start out as a 17 year old white belt when you're in a white to orange belt class for people aged 16+. It's quite nice actually... the classes are very small, so you get more individual attention than you would otherwise.
The one thing I would work on, if I was you, is your self confidence. Don't let the belief that you don't have the necessary skills hold you back; no one starts out being able to do something perfectly, whether it's academic or related to sports. If you give up on all your sports because people are better than you (yes they've been playing longer, skill comes with time in almost everything), and you're not physically oriented, will you also drop out of BMOQ (3 - 4 months summer camp of training) because you feel like other people are better? You won't last long at all with that attitude. Sometimes you come out on top, sometimes you come out on the bottom - that's always the way it is because people have different strengths and weaknesses. I'm going into this knowing that I'm likely going to get my 'behind' kicked when it comes to the physical portion of training, but I'm doing my best to catch up. Other people won't be as strong in academics. They'll help me improve when it comes to the physical portion, I'll help them improve when it comes to the academics portion - teamwork, you know? Teams don't work really well when their members give up.
Anyway, I know the last bit is a bit preachy, but I hope you'll take it in the spirit it was meant - encouragement rather than discouragement. I wish you all the best, regardless of what you choose to do.
(Edited to fix website address mistake.)