Out of all the talk on double slit experiments, I have never heard anything about how electrons or light interact with the material of the slits. I am use to using RHEED/LEED (electron diffraction) in semiconductor manufacturing, so could the patterns just be electron diffraction off the atomic lattice of the edges of the slit material? Then you get an interference envelope from the 2 electron diffraction patterns from 2 slits. If it is just electron diffraction off the atomic lattice of the slit material, then even if one electron at a time, you naturally get a gradual build-up of a diffraction pattern off the atomic lattice. Nothing mysterious. Maybe no need for some 'spooky' quantum explanation?
Also, I think it has been shown (by an Italian group) that when an atom/electron detector is there, usually in front of the slit, it of course disturbs such electron scattering, so the pattern changes. Again, seems nothing 'spooky'.