I know that the number of people playing Bloodborne is dwindling, but for those of us who are still trying to get eyes on the inside:

I was practicing my speed-runs through Y'hargul (Blood Moon). There's a fairly well-known skip (easily found on YouTube, that lets you bypass a huuuge annoying chunk of the area.
Spoiler
Show
Basically, from the lamp,
you walk toward the elevator, where there is a Saber-guy and Wheelgunman. Just to the right of the Wheelgunman,
there is a piece of fence you can clip through with a charged R2.

The thing with this skip is that you fall to a very small platform with a plunging attack to keep you from falling off. Even watching speedrunners, they waste so much time falling, realizing the fall is bad, quiting back to the menu, and reloading to try again (sadly, this is faster than letting yourself die and walking back).

But, I found that, if you do this skip, you will sometimes fall behind the walls and land on an invisible floor, inside the walls next to the Bellwoman. You can then walk out-of-bounds behind the rockin' stagecoach and all the undead almalgams.
You can then walk through the wall at the near the Tonitrus steps.

It took me hours to get the speedrunner trick down, and I still only get it right 1/10 times. But, with a Beast rune equipped to mitigate fall damage and a bloodvial, I can pull this off 1/3 times.
It's not perfect, but it's easily replicated.