Diffraction is something that I've never really worried too much about. The theory may be that it'll make pictures less good than they could be but what matters to me and maybe what matters to others too is how the final picture looks and if the theory says that diffraction could be an issue but the final picture looks ok... then who cares about diffraction
I've taken pictures at apertures of and smaller than f16 with all of my cameras and I can't say that anyone has ever said "Oh er. Diffraction there mate!"
And on the general subject of how much we need to know.
Once upon a time I used to fix stuff... sometimes we'd be sent to fix a thing we might not have seen before and we just had to get on with it. I worked with a guy who had a different approach to me, he'd try to understand the thing and how it worked in the belief that understanding would help him to fix it. I had a different approach, I didn't care how it worked or if I understood it or not, I only cared why it didn't work and what I had to do to get it working again. Occasionally back then I had to engage my brain and work things out but most often a deep understanding of the finer points just wasn't required and I do believe that the same is mostly true in photography. I do think that we need to be vaguely aware so that if we need to we can do some deep thinking or go away and research and learn but mostly I think that photography is technically pretty simple. See something worth taking a picture of... aperture, shutter, ISO (there's a contentious one
) perspective, framing and composition... Isn't that enough a lot of the time.