Alternate? Who? Iris loves Mizuki. Mizuki and Nagisa love each other. There's been no indication Nagisa would love Iris even though he sacrificed himself for her sake all the way back when he was a little kid. A normal person might be even resentful, but I reckon since Nagisa dedicated himself for the revenge against the Kiryuu family, he would hold Iris as a kind of impersonal justification and raison d'etre, and after all these years not feel much else. His existence was kind of flimsy, at least until he realised he loves Mizuki and might also live for himself, not only for vengeance. Like I said before, I suppose it's possible Iris would now suddenly feel something for Nagisa, but I'm not sure you could honestly call it love instead of gratitude, pity, or who knows what.