Nikaah & Marriage Issues

Marrying one's step brother's daughter

Q: I have a step brother (same father, different mother), one of his daughters have been recently widowed. I myself am divorced recently as well. A few people have raised a question about us getting married since my widowed niece is just half niece.

I am clear on the ruling based on Quran (Chapter 4, Ayat 23) that this not allowed.

Can I please request guidance on this please?

Getting married without a wali

Q: I am about to turn 19 and live in Canada. I am baligh since the age of 9 (which means I had my periods). I am currently in college and I like a guy. I want to do nikkah but my parents don't allow nikkah at such a young age specially till I don't complete my studies which has around 5 to 7 years.

I am scared of doing something haram (like zina) and I want to turn it halal as soon as possible. I have the witnesses, I am completely sure about this marriage since the guy belongs to a good family which is also religious and fulfils its Islamic duties. But I recently heard that nikkah without the consent of wali is invalid.

Now my question is, in what Hadith or verse of the Quran is it mentioned that the father should be wali? Can I make an imam my wali? Or can I conduct my nikkah without a wali while fulfilling other conditions such as witnesses, and haq mehar.

A detailed response over this matter for better insight would be appreciated.