I have to read your message very thoroughly, but at a first glance it looks like you will be able to do such things by simply assigning a math font set with different settings to the \textfont0, \scriptfont0, \scriptscriptfont0 (as all math parameters depend on the math fonts).
The main difficulty is not the style settings for each of these fonts but the \text \script \scripscript scheme of sizes iself. Note: -- The numerator and denomiator of fractions come out in a smaller size than the normal one, approximately what would be the \smallsize of LaTeX. -- Fractions within fractions appear at same size that the main fraction. -- Exponents are set in a size that would be the script size of the fractions text, and the same size is used bor both text-size basis and fraction-size basis. -- There is not a subsubscript size; i. e., all levels of sub/superindexes are displayed at the same size. Thus, the size of fractions is not textsize neither scriptsize.