So, I have tried myself and it works for me. This is what I did, could you confirm this is also what you have done?
--SPOILER AHEAD---
-I downloaded the second file
-I got all common dolls
-I took scissors and matches to kill the broken dolls and Okiku-san
-I got the yandere doll and sent him away by talking to him
-I got Mirror Helen
-I chose true end
-I watched the animation and opened the dolls' room following the order on the sticker
-I went upstairs and clicked on the red book
-I chose the option "I don't have any wish"
Then I went to check the stairs next to the Japanese door and the yandere doll was there.
Just in case, I'll create a new download link with the stairs unlocked so you can play it at least :)