You've asked me for truthful advice, so I'll try not to sugar-coat anything.
I've recently been forced to re-consider my views on the importance of GPA in getting into medical school. Previously, I've been of the belief that personal characteristics are what get you into medical school. However, while your personality might be the key to performing well in the interviews, the reality is that you must first get the interview.
I've had the rude awakening after this year's application process when two good friends who would both make excellent doctors did not receive interviews because their marks weren't high enough. The lesson here is that extra-curricular activities, an engaging and sensitive personality, and a personal belief and drive towards medicine don't mean anything if you don't get the interview to showcase your non-academic side. I've re-written parts of my web-site to reflect this change in my perspective.
Accordingly, since you've obviously done a great deal of medical work, both paid, and volunteer, you really need to focus on the grades to make your application more well-rounded and balanced. Your first priority must be to raise your pre-requisite GPA. A pre-req average in the low 70's is stretching the lower end of the admissions spectrum, considering that the average accepted applicant usually has an A- average or so. If you really turn on the jets and do extremely well in both third and fourth year, I do think you have a chance at being admitted. It might be tough getting into US schools (the public ones anyway) as a Canadian student because your GPA isn't that high. If you were to write an extremely strong MCAT though (say 35+), that could all change. So...
The second way to prove to the admissions committee that you can academically handle med school is to do well in your MCAT. Is there any way you can move that summer course to a better time? If not, and you're set on applying after fourth year (so you MUST write the MCAT in Aug 2001), I do think you could be prepared for the MCAT if you were to study hard, and I mean hard, starting in early June.
It certainly wouldn't be easy though. But if you're motivated enough, I think it could be done. Finally, I don't think the W will affect your chances.