How were early Muslim women treated? What factors might have caused this to change after the spread of Islam?

To answer your first question, early Muslim women were treated with respect and had the same amount of rights as men. for example, they could own property, gain an education etc. for example, Khadijah the wife of Prophet Muhammad was a very successful businesswomen in trade.

second question: as Islam spread, people intertwined their culture with Islam. for example some people believe that in islam women are not allowed to pick their own husband and instead have an arranged marriage. however this is a cultural belief, common in places such as Bangladesh and Pakistan.