Why did Annabelle say that Wren could stay with Hugh when they split up, and why did Hugh think she let him keep Wren?

Annabelle told Hugh that she was giving him custody of Wren, because that way Wren could be as comfortable as possible. If her parents were splitting up, at least she got to stay at home with her friends and father. Hugh believed that Annabelle had given him custody because she felt guilty. She knew she had cheated, so she had given Hugh the best part of their marriage.