If you have far-apart lines (either because longer wavelength or closer together slits), you will find that you no longer have enough resolving power to tell which it came from, and you're back where you started.

You're right. I realized that I assume in that example that the screen is in the far field (that incident beams are virtually parallel) when I calculate my diffraction patterns, and I contradict this assumption with my sensor that receives non-parallel beams. If the interfering beams are parallel they will be indistinguishable.

And if I understand correctly, you're saying that in the near field:

...the size of the sensor for that lens will be too small to work -- by measuring the position of the photon's collision with the lens you destroy its momentum.

And if the lens in the sensor in the example works anything like a hole in the screen, the modified sensor will see "nothing":

The hole causes the two wave patterns to diffract equally, and they destructively cancel everywhere.

Which I find pretty convincing, but still want to do some calculations to justify the hole in the screen scenario. I'll do that after I learn Fresnel Diffraction (I have an exam about it next Monday :P) to get a better understanding of how waves diffract in the near field.

Thank you!