The way I see it, Manbagi's greater regret is not that the person she was interested in was taken, but rather that she never mustered the courage to confess. So in order to grow and move past that, this time, she will confess to Tadano, with or without Komi's understanding (though I think she will tell her).
For Tadano, the trauma of an earlier rejection was so great that he doesn't feel that anyone could care for him. Manbagi's confession might be the catalyst he needs to finally face his own feelings and for once, think of himself before thinking of others.
And if Manbagi tells Komi about her intention to confess, that might drive Komi to also find the courage to express her own feelings to Tadano herself. It's all Endgame from here on.