A little spoiler. Kunio = Kasumi. You need to do a couple of quests before getting Kasumi. Refer to this walkthrough my William and Mr. NTR. https://itch.io/t/5475564/walkthroughguides-with-image-crimson-bird-games
For Sasaki you need to have sex with her first then she will be able to join you.
Nurse exist in different timeline. You can switch timeline by helping Mrs. Yamada in the past. There's a portal in the blank room which will take you to the past. Find her on third floor girl's toilet. Talk to her. You will be hinted to talk to Mrs. Yamada in the present. She will tell that she didn't had sex with boy but she would have. After that go back to past version of her and she will ask you a question. Answer anything as both are incorrect option. When you try to go back you will see a girl in the next room. Talk to her and she will tell you to go back to Mrs. Yamada for another chance. With the help of that girl answer everything correctly and have sex with Mrs. Yamada there. That should shift timeline once you exit the level with the computer in the truck. You can verify timeline shift by going to the asylum and seeing there's no mention of Mrs. Yamada in the asylum any more. There's a lady with the name Misa there. Now catch the ghost and find nurse in toilet I think. Make sure you don't choose a flattering option if you want her ghost to hunt you.