Yes! (I wouldn't recommend applying after two years because your odds are so small). Except I'd apply after third year with the expectation of not getting accepted. Only 8-9 people in my med class of 120 got in after third year. While that may sound discouraging, I would look at applying after third year as a trial run. It forces you to write your autobiographical essays a year early, and helps you identify areas of your application that are lacking.
In the event you get interviews, it's a great chance to meet the schools, exchange gossip with other interviewees (you'd be surprised how much medical admissions info makes its way around), and polish up on your interviewing skills. Who knows, you might even be accepted! But, in the event that you aren't, you'll be far ahead of the game when you apply in earnest after fourth year.
Most schools have a limit on the times you can apply, but it varies from school to school, so you should check up on that. Most schools allow you to apply at least three times; if you still have't gotten in after that, then you probably need to do some serious re-upgrading on your application or look to another profession that interests you. I wouldn't be worried that you might somehow "waste" an application attempt by applying after third year. Med schools shouldn't hold any bias, or penalize you when you re-apply; UBC doesn't. Still, I'd e-mail the individual schools and ask.
For doctor-shadowing programs, try calling the volunteer office of your local hospital. Also ask your friends and relatives whether they know any doctors who would be willing to let you follow them around for a day or two. Most doctors still remember the "med school days" and may be very enthusiastic.
You're probably doing just fine in the volunteer department as long as you're actually getting something out of the experience.
Co-op programs probably don't make you look more favourable than not, but if you are in co-op, you get to travel, meet tons of co-workers, and most importantly, interview lots! Interview experience really helps settle the butterflies, and lets you answer questions with poise and confidence, which will serve you well in the medical admissions interviews.
Finally, I've just finished first year med, and am going into second year, the last year of lots of classes before we hit the hospitals. I'm looking forward to second year. :)