new muslim

Q:

1. Is it necessary for a new Muslim to publicly profess their Imaan?

2. Under what circumstances will it be permissible to hide one's own Imaan?

3. If a person dies hiding their Imaan, how will the Muslim public ascertain that this person was a Muslim? For example, if a person continued to live with their non-Muslim partner; did not dress as a Muslim; did not make a request for a Muslim funeral and did not change their name, would this be sufficient evidence to not classify them as Muslim?

Q: I got a question about marriage. I have recently met a girl that has reverted to Islam, and me and her we want to get married, I am a muslim, my parents approve, but her parents are non-muslim and do not approve her getting married. So I was wondering who will be her Wali, as nobody in her family or house hold are Muslims. How do we do it with witnesses when we are doing nikah and she is 18 so its legal.