You answered it yourself. Marco was too strong for any King vs Marco battle to have any kind of tension, so he needed to find a way to weaken Marco, which is where the Marco vs King and Queen plot(while protecting samurais with his blue flames taking a toll on his stamina) became important to increase the tension in the Marco vs King battle
And Oda may have also added the Perospero plot to shoot arrows in Marco's back the time Nekomamushi comes to deal with him, for the Marco vs King battle tension to be complete. Marco will still beat him though