He's not protecting the plot, is just that the author write things in a way that Souta decision ends up being the correct one.
In the beginning, he chose to not interfere with the Hero's team (because of the plot) and thus ended up managing to avoid Tsukishima's attention.
In the Exam, he went...
He's not doing that to save the cast if you pay attention to what Risa said eariler he was the one who sabotaged E class and did nothing on the exam either. He's doing that to impress Kaoru and build his own harem. He doesn't give a rat ass about his classmates.
Unfortunately, Kaoru is the mangaka favorite heroine so she's not going to cuck Souta or show any sort of interest in another male character, I think it's obvious.
Also, I don't expect Souta to romance the other heroines as well.
It's the mangaka choice to give focus on them, in the novel they don't show much because we see more focus on Souta's adventures. Mangaka sacrificed those to write an original story with the hero's party and give them character development.