At least in Fatal Frame 2: Crimson Butterfly, why are there working lightbulbs? It was something I noticed the first time I played the game. I can get with candles being lit around (which is also strange but ghosts...), but there shouldn't be any electricity in the village.
If ghosts can cause candles to be lit, why can't they cause lightbulbs to turn on?
Actually, those are lanterns.
Why were Mio and Mayu lured to the Lost Village? What connection do they have to the village? Yes, they're a set of twins and the ritual uses a set of twins, but why them?
Honestly, it probably comes down to 'they were nearby' but one of the supplemental books states that their father had also been trapped in All God's Village. It's possible somehow that led to the twins getting trapped.
What about the ritual itself? Were the sacrifices really keeping spirits from leaking from the Hellish Abyss or were they sticking to that belief?
Personally I think it's Your Mind Makes It Real. They thought there was a Hellish Abyss to keep at bay so a Hellish Abyss started forming.
While the protagonists save the main villain, save the spirits and purify the area of the malevolent force from the Underworld what is preventing from the same disaster from happening again? While Kirie is keeping the door from opening again and the Rift is contained what is preventing the Hellish Abyss from spewing out the Repentance again, the Day Without Suffering and the black water from overflowing again? It is thanks the Crimison Sacrifice and the creation of Pillars in their respective game from causing these disasters from happening again and there could be someone stupid enough to do the Kaigoru again?
What is Getsuyuu Syndrome? Given the notes found, it sounds more like it's a mental condition and not an (actual) physical illness. Actually sounds akin to Dementia or Alzheimer's, as it makes the person lose their memories and their ability to recognize their own face. So, if it's labelled as something that is more a mental condition, why would it have a physical symptom like Blooming?
Both dementia and Alzheimer's do have physical symptoms, beyond the fact that they are a physical problem with the brain.
More referring to the fact that it feels less like a, well, 'real' disease and more just a mental thing. A Your Mind Makes It Real thing. It's not a 'real thing', if you just think you have Getsuyuu syndrome long enough, you start showing signs.
If the Camera Obscura was created before film existed and it's said to not even use film at all, why do you keep having to pick up film and using it when using the Camera Obscura?
Considering that FF's Camera Obscura was invented some time in the 1800s (about 1850-1860sish), film would have existed (photographs do exist from that era), it just wouldn't have been commonplace, however, I'm guess it's not really film per se, rather, the filament. If it's not the filament, then it's possible that why it was said to not use film was a translation error.
The creation of Yomiko/Shadowborn. They're children born from a ghost and a human parent. How can conception happen, if one parent is dead? The ghosts do seem to have physical enough 'form' that the protagonists can touch them, but... they're dead! There is no biological way this would be able to be pulled off. Ovulation doesn't occur anymore with ghosts, stiffies and sperm don't work anymore... Is it less physical and more ritual-like based?
Probably physical in a supernatural sense, so most likely it's ritual based.
What happens if a set of twins consisting of one boy and one girl are born in the All God's Village?
Going out on a limb here, but they might be seen as unfit for the ritual. Outside of the village still using the old way of deciding which twin is older or younger, they might also have the old way of thinking that non-identical twins may be a reborn couple that committed suicide in the past.
I presume nothing, as, the ritual calls for identical twins, so one could assume that they just live life like normal, if not used in some other ritual, supposing they weren't killed after birth.