Sounds good. What I've mostly seen is that LDL, total cholesterol, and triglycerides seem to be highly correlated, and adding in any one of those three (assuming you also have HDL) gives the same accuracy, so the risk modelers usually just stick with total cholesterol since more patients know it.

For ratios, this particular model is log-linear: risk = b^(w0 + w1log(cholesterol) + w2log(hdl) + ...)

So adding in variables to represent ratios would change the coefficients, but not the final risks.

For that last one, the "low cholesterol" bucket they allude to is people whose total cholesterol is <180 mg/dL. 180 mg/dL is exceptionally low, particularly for an older population. There's also a potential confounding effect when you study only total cholesterol (ignoring HDL)--your "low cholesterol" group of people includes a disproportionate number whose HDL ("good") cholesterol is also low, and who thus are actually at a high risk of heart disease according to models like the Framingham one.