I agree that graph I posted leaves some things to be desired. My impression is that it's trying to portray more information than can be reliably portrayed in a two dimensional graph, and it's probably more of a hypothetical portrayal of a typical sequence of events than anything else.
The other diagrams on the page required some study before I felt like I could understand them. It helps to recognize that there are three sets of valves represented in those schematics, described as "In", "Out" and "Cut". In the absence of electrical supply, the In and Out valves are closed and the Cut valves are open, leading to the arrangement that appears in the diagram labeled "3.Brake malfunction". This appears to portray a conventional hydraulic/mechanical braking system, and it's what you would have if a fuse blew at 60 MPH and you needed to use the brakes to stop (one hopes that all of these valves will be 100% reliable in opening and closing for the lifetime of the car, or things might not go so well in a power-out condition).
My take after studying the diagrams is that when everything is working correctly, it is 100% "brake by wire" and only diagrams #1 and #2 come into play. The computer regulates the In and Out valves to modulate the pressure, based on some sort of "Driver's demand" pressure signal, the sensor for which I could not confidently determine from the schematic. But in a power failure scenario, we have diagram 3 and it's a conventional braking system.
As for rust on brakes: I live in an arid climate so don't have many opportunities to investigate this, but I recall an occasion years ago where I happened to be looking at the rotors of two cars that had been parked out in the rain, on grass, overnight. Both cars had been driven the previous night and I was checking them out in the morning. One had rotors without a spec of rust, the other had rotors that had already acquired surface rust, in the perhaps 10 hours since they had both been parked in a damp environment. The former was a BMW, the latter was a Mazda RX7. Both nice cars, but the BMW clearly had higher grade steel than the Mazda with respect to the material that the rotors were made of. Which begs the question: what happens to a Niro's rotors if you leave it outside over night in warm weather and damp conditions? And if they do develop surface rust as quickly as that Mazda did, then that might actually be an interesting way to discover how much you are using your friction brakes in the course of driving, in that if you drive and stop a few times and it's all polished off, then you're probably using the friction brakes a fair bit, and if it isn't, then perhaps you haven't used them much at all. Not quantitative, but still possibly offering insights.