Danzo's an idiot.
He really isn't. He solo'd Shisui in a few seconds
probably. I'm sure Jiraiya was well aware of the dealings Danzo had with Orochimaru, his former teammate and fellow Sannin. Since he didn't want to be Hokage they went with the next best choice, Tsunade.
he want to show that girls can also become hokages
I find it strange Danzo didnt use KA at all to become hokage