In the version of the game downloaded from this site (Steam version/World version), almost half of the characters don’t appear in the gallery, and I never encountered them in the game. They only showed up when I downloaded the game from another site, where the translation was done with MTool.
I just checked, the only "hidden" girl is Yurika, she comes from another game. (Asaka also comes from another game but she's not a hidden character, she comes in a DLC for the game)
Your game has all the girls, you probably just haven't seen them. If you've reached the end of one playthrough and started another (continuing from the previous one) and still can't get the girls, you probably have some mod installed that's causing that error, or you have an old version of the game registered without the DLCs or Yurika.
To unlock Yurika without mods, you need a file located in:
C:\Users\NAME\AppData\Roaming\ベルゼブブ
The file is:
超速くノ一クロユリ.dat
Just extract the file and place the contents into the folder.
The rar contains files from other games by the developer; if it asks to replace files, say no.
After you have
超速くノ一クロユリ.dat in the folder, open your game and in the game window, click on the top left: File > Delete Work Information. Then do the opposite: File > Register Work.
Since you've tried so many versions of the game, you probably have an older version without the DLCs and without Yurika registered. So even if you open the complete game with all DLCs, if you have a version without DLCs and without Yurika registered, then they won't appear in your game.
I hope my explanation isn't confusing.
Chousoku Kunoichi Kuroyuri is the game of Yurika.