1- Not not only when their husbands died. So they were captured to be moved from the corrupt villages of polytheism and taken to a healthy society like Medina. Secondly, after being enslaved for a while, they’d learn how to take care of themselves in Medina. They learn how to function in the city, make ties, and get support from the state or generous Muslims.
2- As for why the master did not need permission to have relations with her, I did answer that in the video. Islam wanted him to have relations with her so she would become pregnant and have child from him. This elevated her rank in society and forced the master to treat her like his own family. And she would be on her way to freedom. As soon as the master dies she would automatically become free if she had a child from him.
So it was in her interest to all the master to have relations with her. In fact, during those times, female slaves wished they’d had a child from their master so they would be treated better and not viewed as a slave anymore.