Her reason for not dealing with the Juubi if she was the "strongest". If she stopped war, that means she wanted some type of peace. But The Sage brought peace, not her. It seems she ruled by using Pain's logic. Peace through fear.
Again, to call her the strongest is still an overstatement. Then it leads to pointless speculations.
Again, to call her the strongest is still an overstatement. Then it leads to pointless speculations.