I took the MCAT in April without having the second half of the last quarter of physics (electro and magnets). I guess that'd be pretty similar to your situation. I was able to learn circuits and stuff on my own well enough to be able to guess intelligently (because that's really all you can do) on the MCAT. I got an 11, which isn't bad. So, yeah, it can be done if you're willing to read ahead and learn things without the aid of a professor.
As for your EC's, it sounds like you're on the right track. I personally don't think too highly of school activities such as clubs. They always seemed pretty lame to me, but maybe that was just at my school. But, just do what you like and what you think will give you the best experiences. It's better to have lots of experience at one place than little experience at many places IMO.
Good luck,
Adam