I don't think the marriage was necessary, but it's better than other means because it serves her purposes better. It lets her get two things done at once, it's efficient.
Because there isn't any scheme here, she's being very upfront. She wants an alliance, yes, and she's getting one, the marriage works for that. She wants her new vassals to grow up and toughen up and she's getting that too, the moral and psychological effects of the marriage work for that too.
It's like "The woman you loved just died, tough luck Harry, there's work to do", "The man you love is with someone else and there's nothing you can do about it, again it's not you, tough luck Molly, you don't get what you want" "There's work to do, your happiness doesn't matter, what matter is what's necessary".