The ochric epipedon thickness/depth is not defined by any diagnostic features or qualities it possess, but rather by failing to meet the diagnostic requirements of any other diagnostic epipedons, or of a diagnostic subsurface horizon it might overly.
The lower boundary of the ochric epipedon extends through eluvial, E horizons. Or to the upper surface of any diagnostic subsurface illuvial horizon. If the underlying altered horizon only meets cambic or oxic criteria, then the lower depth of the ochric is the depth of plowing, or in unplowed, then 18cm.
I don’t know whether the exact meaning of ‘appreciably darkened” has been defined, but I take it to mean “darkened sufficiently by humus to meet the color requirements (value/chroma) of umbric or mollic epipedons”, but said with a lot fewer words. I don’t believe ‘darkened sufficiently’ implies anything about meeting the depth or other requirements. That phrase is only used once in ST and KST, in the one paragraph quoted above. I think it means “meets color requirement”.
In your scenario, I believe the O and A horizons would be considered the ochric epipedon. You didn’t propose what diagnostic subsurface horizons, or substratum, it may overly.
The Munsell Soil Color Charts do not represent numeric values of color. The hue, value, and chroma are simply reference point on a color wheel, not integers. For example, 7.5YR 3/3 is simply a notation, not a numeric value, so mathematic operations should not be performed on soil color. That’s is, a ped with 5 YR 2/2 color plus another ped with 10YR 4/4 doesn’t average to a 7.5YR 3/3. Color is determined by whole chip only. The chip closest to matching the soil in question is the color reported. Rounding, averaging, and weighting are not appropriate with Munsell soil color notations. If you determine soil color in several locations within a horizon, some domains might meet mollic, some might not, but the dominant occurrence is reported as the matrix color, so if 2/3 of the locations meet mollic and 1/3 do not, then the color requirement for mollic epipedon met. I don’t believe you could determine ‘mixed colors, by averaging the color value and chroma notations.
