For the sake of discussion and maybe I just wasn't able to pull it out of the rest of your post but why would you think that Rom is not mindless? Seems weird that they would go through the trouble to label him as mindless.
I played through the game twice and I still feel a tad lost so this could be all on me.
Well, the major points would be:
- Vacuous itself isn't necessarily mindless, it could also mean inexpressive, devoid of purpose. In the brazilian portuguese version, she's actually called "Rom, a aranha inexpressiva", so, Rom, the inexpressive spider.
- If she was indeed mindless, stupid, why would Micolash still wish to ascend as she did? And he's not just unaware of her state, he specifically says "as you once did for the vacuous Rom, grant us eyes". Seems to me he believes Rom isn't stupid, just didn't share.
- The following lore note, found in Byrgenwerth: "The spider hides all manner of rituals, certain to reveal nothing, for true enlightenment need not be shared."
My understanding is that Rom successfully ascended to become a Great One (maybe not a true Great One, but regardless, that is another discussion), but then, nothing. All the eldritch knowledge they were expecting to get by one of them becoming a Great One was kept from them, since the spider shared nothing. The vacuous title almost seems to me that it was given out of spite.
We can speculate on the reason she didn't share the eldritch truth, maybe by becoming a Great One she was so beyond her humanity that she didn't care, maybe she was doing so in order to protect them, some secrets are better left untold, or maybe she just really is incapable of sharing. We can only make out a few words through Caryll Runes, after all, and the Rune Workshop description hints that Caryll only started his work quite some time after Willem's time (he's still somewhat alive, but he seems to be actually vacuous).
Micolash wanted Kosm to do to him what was done to Rom, because that was the only way for him to achieve this truth, he had to ascend in order to understand it, the spider was certain to reveal nothing.
Why does the moon appear so close to earth?
That's just part of the thing, I guess, haha.
Relevant lore notes:
In Old Yharnam: "The red moon hangs low, and beasts rule the streets. Are we left no other choice, than to burn it all to cinders?"
In Byrgenwerth: "When the red moon hangs low, the line between man and beast is blurred. And when the Great Ones descend, a womb will be blessed with child."
Relevant to the previous note, in Yahar'gul:
"Madmen toil surreptitiously in rituals to beckon the moon. Uncover their secrets."
"The Mensis ritual must be stopped, lest we all become beasts."
It's part of the night of the hunt, the moon gets closer, and the Great Ones descend. People go mad and become beasts, the Church conducts their nightly hunt, and when the sun rises again, things go back to normal.