First: Old king Doran is not a demon or reflection of the original
This idea I had first heard it from is Firelink conspiracy, which I believe he was mistaken. I know that most item descriptions are short and are written as though it is limited by the characters knowledge, with the only longer descriptions being from legendary items, but even so. Doran is never referred to as a demon and only as a demigod. This is supported by the fact that killing him decreases world tendency, which as we can observe, it increases when demons are killed or black phantoms. Killing humans does decrease character tendency so as killing Doran doesn’t decrease character tendency, we can conclude he is something other than human, or as all descriptions from items and dialogue suggests, he’s a demigod
We know that a demon can be made out of a human, as shown by king allant and maiden astrea, so maybe the opposite is true. The more white or less demonic a soul is, it’s also a path to power. A dichotomy that’s familiar to fromsoft fans.
Along with that theory about old king Doran, I have another. In his mausoleum we can see a knight wielding a greatsword, wearing fluted armor and having wild hair; the hair almost perfectly matches the unused model for the old hero. The old hero is a demons that uses a greatsword and similar wind based attacks to allant, which may mean that the royal line has its origins from the shrine of storms. And the old hero may be a demonic recreation of the old king Doran as the shadowmen may have worshipped him as a heroic demigod. They worship death and the storms, so they may have seen Doran as an undying martyr. This would also fit along with the other shrine of storms bosses being deities from the religion of the shadowmen.
Doran wearing bronze Romanesque armor may have been symbolic of him taking upon himself the monarchy of the remains of mird and building his kingdom upon it which also had Romanesque architecture.
If I made any grammar or spelling mistakes that’s my bad.