Why is he more intent on convicting Ed than in pursuing and then later simply entertaining Frank's guilt?
The issue really is not a matter of entertaining Frank's guilt or even anything to do with the resistance. The issue is that Kido was going to have to commit seppuku, for not finding the person who shot the Crown Prince. He was in the middle of performing the ceremony when he was notified that they had found the gun used in the assassination and had the assassin in custody.
When Frank eventually starts to feel guilty and goes to try and exonerate Ed, it is too late because Ed has already signed a confession, thus saving Kido's life even though he was not actually the one who did it. The Japanese leaders are happy that Kido was able to get a confession and they can close out the case and gain favor in the public eye.