I see some difficulties in answering this question (Have we really measured the wavelength of light?) about the measurement of the light's wavelengths with double slit interference.
"... we can find wavelength, because the way that waves interfere is reliant of wavelength." is the usual answer. This is an possible evidence through some calculations of the measurement of the slit's width and the distance to the observation screen. The direct measurement of wavelenght or frequency isn't performed.
Fringes which we use for the calculation of wavelengths occur behind a simple edge too and this can't be explained by interference from two slits and their distance. To find the wavelength you need the slits distance and this is not existent for an ordinary edge.
When one bring two edges close to each other the observed behind the edge fringes occur again (at some distances of the observation screen). Now one take a second slit and bring the two slits close enough together and the fringes again occur in the same distances. Amazing is the opposite doing from double slit to single slit to a single edge. No slit distance for the calculation of the wavelength but the same pattern distances.
If a measurable phenomenon (fringes distance) exist without a second measurable phenomenon (slits distance) and one need for the calculation of a third parameter (wavelength) the first and the second parameters, is it allowed to give the third parameter a physical reality?