It would've been a nice touch if they didn't so much focus on the Nazi regime, but instead gave some in game reasoning for the relevance of WWII. i.e. the effects of "Chaos" spreading throughout Europe, beyond Wallachia, even worldwide. With Chaos rising, the hearts of x% humanity become tainted causing people to become paranoid, desire war and destruction, and realise the coming of their Dark Lord.
The Portraits then act as Wormholes which spread demons from the underworld all over the globe.
Resume og story of Jonathan and Charlotte arriving, Brauner severing the link to the throne room while claiming the Castle's throne in his own "temporal space" accessible by his blood art technique. (which is a better reason for Death not being able to find him, rather than just entering his portrait anyway, which he potentially could have done from the getgo). Jonathan and Charlotte are only able to access Brauner's temporal space when the power binding the seal (from the other portraits; spaces in all corners of the world) have been severed.. However, they also require the assistance of Stella and Loretta to "open the void"... etc
The Nation of fools could have been the Holocaust occurring, the walls peppered with blood, graffiti and Nazi Swastikas .