Uniqueness

In your opinion, are all the laws of yichud, such as yichud with two people, in the city in the field, during the day and at night, with her husband in the city, etc.? Also, the criteria are informal and everything depends on a person's assessment? Is there some clear criterion that the Sages forbade, namely, a person who is convinced that even in complete yichud he will not be prohibited, is yichud permissible for him?

https://etzion.org.il/he/talmud/seder-nashim/massekhet-kiddushin/%D7%A7%D7%99%D7%93%D7%95%D7%A9%D7%99%D7%9F-%D7%93%D7%A3-%D7%A4%D7%90-%D7%99%D7%99%D7%97%D7%95%D7%93-%D7%9B%D7%A9%D7%90%D7%99%D7%9F-%D7%97%D7%A9%D7%A9-%D7%90%D7%99%D7%A1%D7%95%D7%A8

In a private house with, for example, 2 floors with many roommates (5 or more roommates), is there some composition (i.e., number of men and number of women) where men and women are allowed together? It's just not always easy to find the same sex, and then there are empty rooms that the payment for falls on the person in the house, and this can add up to quite a bit of money.

In principle, you shouldn't be alone with a woman. There's nothing special about an apartment or house where someone else could enter at any moment. But I don't think that's the case in your case, because there are hours or days when you might be alone with someone else without anyone expected to enter.

Hello,
Thanks for the answer.
If I explain the situation to the neighbor and bring him a key and tell him he can come in whenever he wants 24 hours a day, 7 days a week without knocking, does that solve the situation? And if not, is there another solution?
Thanks

It's hard to say anything clear here. He won't really go in, because he has no business there. I think it's problematic. If there were a separate unit in this house with a separate entrance, that would solve the problem.