Just a parent here, and the answer to your questions are standard:
1) You can, but med schools prefer to see a full, regular load during the year. Thus, they don't want you to take the sciences over the summer or take GE's over the summer so you can have a lighter load Sept - June. But, if you want to participate in study abroad or explore another non-science passion, summer session is a great opportunity to do that.
2) Med schools don't care about major. Just grades and test scores, primarily.
3) UCs don't care, so in general, no.
4) Research in the health field can help, but A's and a great MCAT is 80-90% of the game.
5) No. Any top 50 college will prepare you well for med school and the MCAT. While SD may have more opportunities to participate in research, it also has more students who are premed (or bio eng) applying for those research slots. In general, Cal will be the most competitive for grades, with Irvine least competitive. But that being said, you'll do best where you'll have the most fun and enjoy your four years.