JT26=>
I'm still waiting to go for stage 2, so don't think I speak from experience, but if you read through the thread, there are people who got onto the course without flying experience. However, I think it's worth giving it a go if you have no flying experience at all. I got a bit bored while doing my masters this summer and just went for a trial flight on a whim. It was just fantastic. It also convinces you that flying is the career for you and gives you determination to fight for it. I also thought they'd ask in the interview why I never bothered getting at least some flying time and there's no real answer to that.
I can't say whether your grades'll put you in a better position in terms of selection, but they won't hurt your chances.
Have a look at the CTC website for your airline related questions. You get to turn down an offer of employment, but only a limited number of times. You're financially tied to your airline for 7 years, but I think it's only financial.