Ask Lao Mao about the ownership of the house

Lao Mao,

I'm living in Massachusetts. My fiance bought the current house before we met each other. He is the only owner of this house now. After I moved in, he asked me to pay for the mortgage with him. He told me after we get married, he will add me as the second owner of this house.

After reading many posts of the forum, I'd like to ask the following questions to protect myself.

1. When he adds me as the second owner of this house after we get married, we will own the house together equally. It doesn't matter how much mortgage we paid each other before our marriage. Is it correct? What's the difference of primary owner and second owner? Can I be the primary owner too?

2. If our engagement calls off, can I ask him to return the mortgage I paid for his house? What can I do now to protect myself in bad situation later? Certainly, I hope everything will go smoothly and I don't need these protections.

Thank you very much for your advice!