So, I had this theory that connected Mad Father, Ib, Witch's House, and Misao.
My theory starts in Witch's House.
In the true end the cat switches bodies with Viola, then switchs bodies with a bald man and will go by the name Ogre, or Onigawa.
Then the events Mad Father happens. Ogre takes Aya's father to a dark world and starts building off of that.
The dark world is the places seen in Misao and Ib.
In fact, the scientist that killed Yoshino is killed by Aya's father, still doing his experiments in the dark world.
In the Truth end of Misao, Ayaka and Tahma stay in the dark world with the baby versions of Yoshino, Saotome, and Kudoh.
Onigawa is probably trying to populate the dark world with things other than monsters, hence why he keeps dragging people there.
In Ib, the gallery is probably an extension of the dark world.
The dark world is probably alot like our world except more confusing and more gory.
The gallery follows the pattern of taking a building, making it bigger, making it a maze, and filling it with gory monsters.
Seeing as the characters from Misao and the characters from Ib got to the dark world in different ways, different rules for exiting apply.
Hence why only the same amount if people who enter can leave the gallery while anyone who is alive can leave the dark school.

That's my theory, I'll probably add more later if I get new ideas, wanna add another RPG into the mix, or need to fix some holes.
See ya!