As I said I don't agree with this theory that killing the spider doesn't change anything, the rest of the story was bound to happen anyway.
The game states clearly that Rom with his rituals is keeping the old gods away from us.
When the moon becomes red a great one child will be born.
When you kill Rom lots of things happen. Red moon finally appears, the Queen with blood on her womb appears precisely in the lake suggesting that Mergo is finally born in the Mensis nightmare dimension and what triggered it were the events in the lake, also you can finally see the great ones in cathedral ward and you get the message "ritual secret broken, seek the nightmare newborn".
I don't think all of this can be a coincidence.
Imo Rom was at least used to prevent the red moon to happen in some way and there are hints that he was a human turned into a Great One with blood experiments, he got the eyes of knowledge of super beings but became mindless, Willem probably wanted to use those eyes putting his mind in control, at the same time he feared the old blood and the consequences of the rituals of Mensis and wanted to prevent a red moon from happening.
So basically we have two dream dimensions controlled by two different great ones, Mergo in the Micolash dream and the Moon Presence in the Gehrnam's dream.
Hunters are clearly used by the Moon Presence and his servant Gehrnam for its own interests. Does the Moon Presence want to get rid of Mergo?
Is there a conflict among great ones?
I don't get what the moon presence wants to achieve, its own blood child? Blood echoes to be more powerful, trying a way to get access to our dimension and rule it?
Also I don't quite understand why if Micolash is now dead as a mummy in Yarghul he's still alive in the dream dimension. If you get killed in the dream you wake up and the great one in it needs an other host (as with Gehrman getting killed by us), but what happens with the Mensis nightmare goes against such rules, the dreamer is dead but he's still trapped in the dream, then both him and the great one in the nigthmare are killed but the dimension continues to exist...wtf?