It's great for you to ask questions, but your questions are best answered with experience. Your questions show you are looking for some objective truth. None of those guys are wrong. Some care more about diffraction than others, that's all. You wont find some kind of universal truth from these general photography guides - they're just guidelines. The only "universal guide" you will find is to not stop down "more that you have to" because of diffraction unless you haven't reached your lens's sweet spot. Diffraction effects can vary based on lens and sensor format, so how can anyone write one guide that rules them all? Some photographers want to get it all in one shot, so you have to stop down, diffraction or not. Focus stacking isn't always an option, say if you have moving water or leaves. No one can tell you how much diffraction is acceptable to you. Why do lens manufacturers make lenses that stop down to f/32 even for crop sensors? Did Olympus and Tamron not understand about diffraction, or did the marketing department overrule the engineers and say, "no trust us, our market research shows ppl will buy more lenses if they go to f/28"? On the other hand, sometimes you may want less dof and the lens has not yet reached its sweet spot. What will you do? Stop down anyway because a guide or review says so?
You have studied enough to get started. It's time to find out for yourself. It's not that you shouldn't ask questions. It's that I don't think people can give you a meaningful answer other than "it depends".
The biggest wrinkle is that most of the technical faults of gear are not even particularly visible at normal viewing distance. How perfect you want the images is, again, up to you. Most of the reviews you are reading are for single samples no one can tell if anything you personally buy will perform the same when it comes to the fine details. The reviews are good because most don't have the discipline or equipment to do their own testing (thanks, crazy150) but rarely represent an absolute truth. It's hard to have a rigorous and academic discussion about something as vague as "decency level."
Sorry the weather isn't working out.
Yeah, it's good advice to go out and shooting, it just came out as if it was bad to compare these things and talk about it with others in the internet. I agree that pixel peeping is not very good spending of time, but also I like to know things before I get to that perfect moment so I don't ruin it by making bad mistakes.
And one point is that this "argument" if f/4.0 gives always everything in focus with m43 (which as a beginner made me think) also made me research and find different perspectives and update my information about it. Others can also read them and enjoy the photos.
From the Don Smith article who says similar things as you, which I liked very much along with those awesome photographs:
"I could show many more examples of varying apertures for landscapes but I think you get the picture (pardon the pun). The key is to ask yourself how you wish to render your composition. Once I get the proper aperture that renders the scene the way I previsualized it in my mind, then I can always change my ISO to get a suitable shutter if movement of any element within the composition is an issue. So think Composition then Aperture. Make choosing the aperture a conscious thought, and if you need a smaller aperture, then don’t get so hung up on diffraction. Don’t let the fear-mongers influence you to the point that you don’t properly record the image at the proper aperture needed to match your previsualization of the finished image!"
I have especially found this true with macro.