The thing about Bloodborne is that most of the story unfolds in a non-physical inner space created by the subconscious, and most of the creatures we encounter--beasts, kin, great ones, pthumerians, even some humans--aren't exactly real. Places like the Tomb of the Gods are merely born of rituals involving noxious concoctions of rotted blood and hallucinogenic materials, but the events that unfold there represent the evolution of thought that the characters experience, which culminates in physical consequences, such as the "ashen blood" sickness, and the subsequent burning of Old Yharnam.
The Hunters are administered a dose of this powerful sanguine drug, which provides them with a fever dream that draws upon their subconscious to fill in the blanks of their conscious mind and find a way to halt the nonexistent beast scourge, while also providing them with the secrets of the past that Byrgenwerth and the Choir each desire. The primary goal they are given is to "cleanse" the blood of the city, a process conceived by eugenics-obsessed, power-hungry madmen. As for the Moon Presence, the being is merely a representation of the Hunters' belief that what they are doing is justified.
Chronologically, it's very difficult to categorize when certain events take place, since some of these events never truly "happened." The Pthumerians can be thought of as an imaginary race that represents the end goal for humanity, which is to achieve peace with their gods and perfect their blood. Instead, it would be wiser to differentiate the PHYSICAL events that take place in the history of the city, then arrange them chronologically, while separating them from the SPIRITUAL events.
For example, I don't think the Pthumerians were necessarily the "first" beings to discover the great ones, but rather they were envisioned by Byrgenwerth to represent the ideal form of humankind, and perceived as "ancient." Physically, the story begins when Cainhurst and Byrgenwerth begin their research into the arcane and seek to perfect their bloodlines. Before "finding" the Pthumerians, I find it more likely that Byrgenwerth stumbled upon the Fishing Hamlet, "which sparked a search for the eldritch Truth from deep within the old labyrinth." By using these powerful hallucinogenics, they unwittingly set in motion the events that would consume the city in utter turmoil. More important than the Pthumerian's alleged relation to the great ones is Laurence's invention of such a fantasy, which leads to further experiments, illness, and ultimately the destruction of Old Yharnam, resulting in the present chaos of the current Yharnam.
Tales envisioned by Byrgenwerth and the Healing Church can be "dated," but similar to biblical parables, they have no place in history, and are simply spoken as faith. Ebrietas is a seemingly ancient great one known as the "daughter of the cosmos," but it seems she only came into existence AFTER the events of the Old Hunters. Despite this, she also appears in the ancient "Tomb of the Gods." Likewise, Queen Yharnam's child, Mergo, was stolen from her by the Choir which becomes the "source" of the beast scourge, yet we can still find her pregnant with Mergo in the Chalice Dungeon, as if this critical event in the transformation of the city has yet to unfold.
Symbolically, the great ones and their interactions with humanity are important, but their effect on the story is merely that--allegory. The Moon Presence does not possess its own will, but represents the gripping belief that the hunter's are justified for slaughtering countless "beasts," even if the beasts are just an illusion. Kos, on the other hand, symbolizes the ultimate truth of the cosmos that the scholars trust will protect them from beasthood, but eventually Laurence discovers that this was nothing BUT a dream, as were the beasts that he sentenced to death, who were in fact just less-than-enlightened common folk. The Healing Church overthrew and executed him for his alleged madness, and desperately clung to their faith in the Old Blood, and the School of Mensis became obsessed with their position of power granted through esoteric revelations.
Thus the hunt continued. Without the hunt, there would be no sense of purpose, and culpability would finally destroy the city.