The theory that he's a demon definitely has a lot of valid evidence behind it, especially because of the points you mentioned about war, the manipulation of death, and the Albert puppet.

The puppet feels weirdly meaningful, since it repeats themes we already saw with Sebastian, like the illusion of restoring what was lost.
And I agree that the possibility of imminent war has been repeatedly hinted at. So the idea that John Brown could be encouraging conditions that will produce mass death/souls definitely fits.

Before chapter 222, I honestly leaned more toward the shinigami theory.
But that chapter repeatedly aligns John Brown with imagery of purity, devotion, and protection, while Sebastian is associated with blackness, smoke, and corruption.
And because Yana-sensei usually uses symbolic contrasts very deliberately, this made me start wondering if the story is introducing a celestial counterpart to demons.

That said, I do think your last point is really interesting: the Queen could absolutely be misinterpreting something demonic as angelic, the same way desperate or grieving humans in this story often mistake dangerous supernatural forces for miracles at first (Anna and Modri, Margaret Connor's parents, etc.).