Are you referring to organic chemistry? Because 112 A/B is the corresponding organic chem series for chem majors, analogous to the 3 series. When I referred to 120 A/B, I meant physical chemistry, which I know is required for chemistry majors and I'm pretty sure it's required for chem bio students as well. Chem 120 A/B is a lot harder than the 130 series, but, depending on the professor, is usually better taught. As far as the science majors being GPA friendly or not, I can't comment on all of them since I don't know enough about them all. I just heard that IB is easier than MCB and MCB's easier specialties are immuno & CDB. As far as the groups being smaller, yeah, 112 series is smaller than 3, but the class still fills up the Lewis Hall lecture room. If you want your professor to know you, your only shot is office hours. Or if you had an awesome special project poster presentation while taking the 4 series and they attended the event.
As far as research, if you're going to be doing research for the chem department you're going to have to sacrifice a lot of your time doing it. I think the reason they scrutinize your motives is because they want to make sure you're serious about putting the time/effort into it. I can't speak to MCB research, but I think most people who are pre-med would rather spend their time studying to get As and preparing for the MCAT than sitting there on the bench tracking a reaction by TLC and spending hours running a column.
ETA:
http://chemistry.berkeley.edu/student_info/USLI/chemistry/#core core curriculum for chem & chem bio majors. In addition to the 120 series (physical chemistry), you have to take more intensive math courses such as multi-variable calculus and linear algebra and physics for scientists and engineers (although not sure if they still do this, they used to accept the 8 series for the 7 series). It all depends on the individual. If you're more physical science/math inclined, go for it and go the college of chem route or physics or engineering. But don't pick any of these majors because you think it'll be "easier" to get As. All of these majors require hard work and are super competitive.