Cavendish is probably younger, but I don't think that is very relevant for the character. It's like changing Morgan's race in the Dresden Files show. I think making her older wouldn't affect too much and we would win with a great actress. BUT if you truly think Mirren is unacceptable, I could go with Emma Thompson. I was considering her for Gwen's mother, but I could put another person there. Cate Blanchet perhaps.
Gwen's case is different. Making her young is important for her character (or so I think). We need young people, like Arya Stark, but I don't see her as Gwen. And Bridget has a different body, I don't see any known star as her, so a newbie will be great.
I am thinking that perhaps the Spirearch could be someone like Eddie Marsan.