Why do you think it's required that the nations need to only make sense for a 5E setting? You seem to be misunderstanding the goals. The goal is that each of these nations holds a place for the core races of the 5E setting. A 4E could for examle include Goliaths, but they aren't a PHB 1 species, so it would also be okay if they are missing. But a 5E setting needs a place for the Goliaths, because they are a core species in 5E.
And the same will apply to the classes - there needs to be patron entities that could work for the Warlock's pacts. If some Fighters can become Eldritch Knights, there might be some places where they are mentioned, maybe a military academy that has a teacher for combat magic, or a mercenary unit known for being lead by an Eldritch Knight.
There should be material that references this to provide guidelines and inspirations, pointers if you were to create your own core rule character on where they might fit in the setting (and also where or how they might feel unusual, if you wish to go against the grain).