I definitely agree with you about getting exposure to the biochem material may be super helpful, and Dr. Nair (the summer biochem prof) is an incredible professor, who wants to see her students grasp the concepts and succeed. However, if you need to retake the MCAT, you need to devote yourself a 100% (but remember to not forget about biostats) and taking the summer course may detract from much needed attention to MCAT. In my opinion (albeit may not be true for everyone), the Kaplan biochem book does a pretty solid job of covering the basics. Going into the Fall biochemistry class with studying the Kaplan book, I was able to recognize many of the concepts and then learn the specifics from the ppt slides. My background was not particularly strong in biochem either, but I did study really hard to ensure making an A in the class.
DISCLAIMER: I know several people were surprised by the difficulty of the Fall Biochemistry course. I do not want to discourage anyone from taking the summer biochemistry course, but I personally would dedicate myself to MCAT studying.
Also, I 100% agree with your statements about LORs from the major advisors.
The prep course used for this semester will be Princeton Review; I'm not sure how exactly that will compare.