Could it be that the silica spheres are actually stacked not
precisely in a plane, but rather some slight parabolic shape that
could in some way concentrate a larger area of a given photon
density into a higher photon density but a smaller cross section?
No. If they did that, they'd no longer properly act as a
diffraction grating to give you single color reflections. these very
fine opals are simply those where the size of the spheres and their
arrangement is most uniform, giving the most effective selection of
the wavelengths, and least loss of light at that wavelength.
Remember that the destructive interference is destroying the colors
you DON'T see. What you see is being quite effectively reflected.
And yes, your ability to see it in dim light is the fact that your
pupils have now opened up and are seeing more light. However, there
is at least a little something in your reasoning that's worth
remembering. The curved front and back of an opal cabochon can act
somewhat as a lens. Cab cut stones often are a little brighter or
more optically interesting than those cut with flat tops and backs.
Don't forget Occams razor. Given all possibilities equal, the
simplest is likely to be true... In this case, there's no reason to
complicate matters. The simple stated structure of opal is quite
capable of displaying the effects seen.