1(a). The most basic answer to that question is that the system must reflect the legal documents. It is a practice all over OpenStreetMap, from the UK to Germany.
In detail, the classification system should be based on the legal numbering system as far as possible (with limited exceptions such as Hydra, where cars are banned). So for example, National Roads on E-roads should be trunk (since I think the definition of a motor-road is too vague for it to have its own category), all other National Roads should be primary, and Provincial Roads should be secondary.
2(b). Navigators should adapt to OpenStreetMap data: there are dedicated tags that give detailed information such as road layouts, condition, surface and lighting (some may know me for having lit up all the classified roads of London a while back). If we were to classify roads based on condition, then this is why the map of Greece looks rather bare at the moment.
It has been something that I have aimed for since 2013, after the discovery of the provincial roads (and OSM was the first major map to number them).