Um, one of your links has statement goes like this:
Genetics and molecular biology are volume II. I guess my question to you is, before you pull what people said, have you taken the real thing? I think not, if I recall correctly.
OP: They are both helpful. Volume II is tough, but real MCAT asks you to analyze physiological changes in molecular level, so you need both Volume I/II. Plus, some topics in Volume II like descriptions of CAC and glycolysis solidify organic understanding, and amino acid's isoelectric points are good for acid base titrations/polyprotic acid understanding.
But, of course, nothing is a must. Do what you feel is necessary.