Than again, your experience is different than mine.
That was pretty obvious in W124 section. Where after checking everything else, the owner would disclosure running on inferior coolant.
Bottom line, your chart might have good use on newer cars, where the problem is mechanical or electrical malfunction.
On aged cars, that we usually discuss in the sections I frequent - the most common problem is chemical build up and my first suggestion is get infra-red and read the radiator temperatures.
Meaning what you put as 7th on your priority ladder, would be the first step for me.
Just real-life variations.
That's fair, but if the rest of cooling system is good already, you pretty much end up rather quickly to the radiator. Chemical build-up can indeed be a problem. But using the correct coolant for your car throughout the lifetime of the car would never result in chemical buildup. The 2 big no-nos in using coolant is 1. Using the wrong coolant, you should ALWAYS be using something that's been approved by mercedes or 2. mixing coolant. You should never mix coolant, if you can't find the coolant that is already in your car, you should flush out the coolant with DISTILLED water. Tap water is not a good idea to flush your old coolant out. Tap water has plenty of dissolved ions that just love corroding stuff, especially if stuff is made from dissimilar metals (like aluminium head + iron block). If you mix coolants, even if its 2 approved coolants, you might end up with unpredictable chemical reactions that form precipitates in the coolant, which usually get jammed up in the radiator. The problem isn't necessarily low quality coolants, this could happen from mixing 2 high quality coolants together. Coolant is not transmission fluid, just don't mix it together. As for using the wrong coolant, you have the incorrect corrosion inhibitors, which could lead to serious problems.
As for the correct concentration of coolant, this is very simple: Unless you live near or in one of the arctic circles, use 50/50. 50% is good enough down to ~-35C, and during summer, the water component can absorb around ~1.5 times more heat per mass of water than the same mass of ethylene glycol (the solvent in coolant). This is due to water's higher specific heat capacity, and if you are worried about the boiling point, remember that the cooling system must maintain pressure, as increased pressure increases the boiling point of your coolant. You should be relying on your cooling system's pressure to push back boiling point, rather than relying on the concentration of the coolant. If your system cannot maintain pressure, it is because there is a leak. Sometimes leaks can be small, so small that they won't affect your cooling system's pressure while your car is running, but you might lose a tiny bit of coolant over time. That's a different issue however.