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?Yeah, agree on parts and have even stated similar, but I ask and investigate certain findings and arguments. There is nothing wrong with that or that it implies I don't do other stuff. You can think what you want about my or others inquiry. I think it's interesting and I learn at the same time.
Btw, the weather in Southern Finland is quite horrid for any kind of landscape photography at the moment at least for my taste.
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.