If you read Sadler's book on Ieyasu he does explain why the Mizuno switched sides to the Oda. I do not blame Mizuno Nobumoto at all.
"A month before this (July 12, 1543) Takechiyo's grandfather Mizuno Tadamasa had died, and his son Nobumoto succeeded. But he was not satisfied with his treatment by Yoshimoto, so he went over to the Oda with his two strongholds of Kariya and Ogawa." (Sadler p. 40)
The Mizuno did play a role in helping Nobunaga win the Battle of Muraki against the Imagawa in 1554. Right after Okehazama Okabe Motonobu attacked Kariya and Mizuno Nobuchika. I mentioned this in a earlier post and in my book (p. 87).
Ehata thing brings up a major question (p. 194), if the Mizuno were allies of the Oda, why the Imagawa attack did not attack them from the start of the campaign? I have some ideas why Yoshimoto did not attack the Mizuno.
- Ieaysu's mother came from the Mizuno. I think their were tensions between Yoshimoto and Ieyasu. The last thing Yoshimoto needed was to stir up a hornets nest.
- Bypass and haul ass! Why attack the Mizuno first when Nobunaga is your objective. The campaign was going smoothly and why change course.
- If Nobunaga was killed in battle, Yoshimoto can then have the tables turned to his favor to convince the Mizuno to switch sides again.
Nobunaga no tame!