Dealing with non-Muslims

Q: Is it allowed for Muslims to live with non-Muslims in the same house?

Q: I want to know if a Muslim and Christian family can live together in one house?

Example: The Muslim family lives in one portion of the house and the Christian family lives in the other portion of the house. Is this allowed in Islam?

Q: My husband and I are disagreeing on something. So basically my husband has friends that are non-Muslim which I dont have a problem with. However my problem is that my husband and them pass their cigarettes around. In my opinion, I say it's wrong as they are non-Muslims and they drink and so on, so their saliva should not touch my husband's mouth. However, my husband feels that I am being unrealistic and it's not their saliva that's touching the cigarettes, it's only their lips. I still feel that's not right. Please advise what is the correct way for this.

Q: Can a person ask a kaafir to make dua without any belief that their idols would fulfil their wish. He believes that only Allah Ta'ala can fulfil the wish of the kaafir and that there is no other God that exists except Allah, but just saying "dua karna-pray for me/us" to a kaafir, would this constitute kufr which takes a person out of the fold of Islam? 

Q: Can I insult a kaafir if he insults Islam?

Q: I want to ask many people ask why we using non-Muslims' inventions practically and socially if we are against them?

Q: Why are we discouraged from taking the kuffaar as friends?

Q: I've been friends with a Christian girl for the past two years. She's always been here for me and so she became my favorite. After sometime, I came to realize that it's not allowed to be intimate with an infidel. What could I do, please?

Q: How should one treat a kafir if that person is your blood relation?