This one is okay, but really just okay in my opinion. I think the problem is that there's altogether too much fluff. The infinite space under the cavern is pretty well pointless; its connection to -2, -3, and -4 is tenuous and underdeveloped, which makes it look like filler.
You have, however, managed a really interesting mental image. Strange device with copper pipes feeding into a young girl. Viscous black fluid pouring from her mouth into her sister's mouth. Identical twins (which is really just a short-cut to creepy town), a poorly repaired farm house, constant dusk. It's reasonably surreal, I'll give it that. Some kind of half-remembered dream or something.
But there are flaws. It doesn't cash the emotional check, for one (an angle it's pretty clear you were trying hard for through the last addendum). I'm a dad, even, and I find it hard to relate for some reason. Maybe it's because you wrote the letter from the POV of a Foundation crony and not the POV of Dr. Banning. Maybe it's because "Dr. Banning" is also the name of the Incredible Hulk. Maybe I'm being a fudd. Nonetheless, it doesn't strike me.
Second major flaw I see is the restricted access. Perhaps I'm jaded or too used to looking at the Foundation from a Level 4+ perspective, but I don't see the need to keep that information secret. It seems to me that it would stress the importance of the classification and the research into repairing SCP-2464-4. Maybe it would make the assignment a bit more stressful, but considering the other kinds of Keter threats the Foundation deals with I don't think that would be a concern.
Despite my misgivings, the prose is solid, the narrative is clear, and most of the choices seem self-consistent and justified. You'll get a 0 from me, but it's an optimistic one.
