The instinct that led you to dismiss this idea originally was probably right. Diabetes and heart disease are both very complex conditions with many factors, some genetic, and, some still unknown, and not at all simply diet-related. The same complex picture is bipolar. If a "cure" sounds too good to be true, it probably is. Caffeine in moderation has protective qualities (e.g. against Parkinson) and, coffee and tea are MAJOR sources of antioxidants in a typical diet. Bipolar is a spectrum disorder and the distinction between bpII and bpI is not even recognized in Britain. The idea that there is a cure for bp2 separately is just ludicrous. There is also what I propose to call overall sanity, which is mightily important. Cutting out all sugar will make you insane if you ever eat out, having to ask the restaurant about each ingredients they use.

To sum up, you will make your life harder and will suffer from a deficiency in antioxidants unless you make an all-out effort to substitute for what you will forego by not consuming caffeine (even dark chocolate, a majorly healthful product, has a bit of caffeine).