One should be very careful when discussing diffraction to make sure you’re about what you think you’re talking about. Case in point: Figure 5. If you look at the drawing, and you contemplate the line segments you’ve drawn, you may find that it’s quite hard to justify the supposed path difference of lambda/2. In fact, the path difference is zero. You need a different drawing for lambda/2, and you might need to relate your drawing to an actual physical scenario (projecting on a wall, for example) to see which approximations are valid.

As for the finger slit, I’m quite suspicious that the actual visible effect isn’t diffraction from the fingers at all — my intuition is that it’s more likely to be diffraction from your eye or perhaps something more complex in the combined system. You would need to extend the drawing to include a light source (perhaps at infinity), the finger slit, and some credible model of an eye including, at least, a finite pupil, a lens, and an image surface (the retina) that may or may not be quite lined up with the focal plane.

Another fun effect: on a sunny day or under a ceiling light, if you close your eyes part way and hold your head at an angle such that you can’t quite see the sun, you can often see fun color bands where the colors change from left to right. This is diffraction from your eyelashes :)