As far as I know, the volunteering requirement is there to make sure you've contributed towards community service, and more importantly, *to ensure that you understand the rigors and demands of being a health care professional.* Admissions committees hate it when they interview someone and receive the following dialogue:
"So, why do you think that you would make a good doctor?"
"Well, I enjoy helping people and interacting with them."
"Well, what experiences have you had that led you to this conclusion? Have you worked or volunteered in a health care setting?"
I think you should emphasize your work in your application. Many pre-med students, myself included, worked in Emerg which makes that type of volunteering neither very unique, nor memorable. Your experiences might help set you apart from the rest of the crowd, in addition to your rather unconventional major. I should emphasize that your course of study won't lead to any sort of bias by the admissions committee, although your interviewers might ask you a question or two about it.
As for your second question, I would certainly be concerned about it, enough so that I would start contacting schools that I was interested in attending.
The reality is that other med schools are likely unaware of the intricacies of UBC Med's entrance requirements. As a result, when you send in your transcripts to the other universities, one of the first things each school will do is ensure that you have fulfilled all of THAT university's medical application requirements. If it isn't evident on your UBC transcript that you have completed a full years worth of Biology + lab, your application might very well be rejected on the spot. I'd try to look at your schedule one last time to try to fit another half year of Bio, whether a first year course, or a higher-level course.
You also might want to inquire to other schools as to whether UBC Biochem 300 can be used to satisfy the Bio requirement, although I don't know if UBC's Bioc 300 also includes a lab; I did my undergrad at UVic.
A similar situation occurred with me back in first year when I was considering applying to UBC's Pharmacy program. I had taken AP English Lit, and received a 4, which gave me first year English credits at UVic. Unfortunately for me, UBC only grants English credits if you received a score of 5 in your AP exam. Net result was that I had to re-take first year English to meet UBC's requirements. The moral of the story is that a credit requirement that is granted at one university is not necessarily transferable to another university. So in your case, I think it would be very desirable to get that second half year Bio course.
I also think you should contact the medical schools you're interested in, and ask them personally, and I would make that my first course of action. I'd also simultaneously register for that Bio course, and drop it later if it wasn't necessary.
Regarding the graduate level courses, I think as long as you are an undergraduate student, any courses you take for credit are reflected in your GPA. Personally, unless you are sure you will do well in those grad courses, I wouldn't take them. There's no sense in being a hero; UBC med has no idea whether a BasketWeaving 500 course is any more difficult than a BasketWeaving 400 course. Take the courses that both interest you, and you can get solid marks in. If it comes down to choosing between those two characteristics, and you really want to go to med school, take the course that you will academically perform best in.