On your usage point: "breast" can mean either "the fore or ventral part of the body between the neck and the abdomen" (as you used it) or "either of the pair of mammary glands..." In a current dictionary (Webster's Collegiate, Tenth Edition -- a very good dic), the latter is preferred. Breast in the meaning of the chest is only a secondary usage now. To shed more light on this, I checked an unabridged Web from the 1930-1960 period. They give the "fore part" def as first, the mammary def as second. So, there has been a shift in usage over the past half century or so, away from the definition you prefer. But neither then nor now is there any basis for saying one is correct and the other is not.
Interestingly, "bosom" also both has singular and plural usage.