In Twelfth Night, Olivia loves Viola (Cesario), Viola loves the Duke, and the Duke is infatuated with Olivia.
Bring out the comedy, and the heartbreak and triumph of true love in these relationships.
This question is even a bit more complex that you have suggested, since Olivia believes Viola to be the young man Cesario. I have added this name to the list you give in your question. It is also interesting to me that you suggest that Orsino's feelings for Olivia are infatuation, but that Olivia's for Cesario (Viola) are love? I wonder how you believe them to be different? And what of Sebastian, who ends the play married to an Olivia he has basically just met? You do not mention him at all in your question of love in the play Twelfth Night. Let's look, however, at the relationships you do mention one by one.
Olivia loves Viola (Cesario). The comedy here comes mainly from the mistaken identity. Since Shakespeare gives the audience the extra information, as they watch Olivia "fall for" Cesario of the fact that Cesario is actually Viola, there is much comedy to be made from this dramatic irony. It is hard, I believe, to treat Olivia's feelings as ones of deep true love, given the comic set-up from which they spring. I would term her feelings for Cesario an infatuation that allows the audience to find humor in her actions.
Viola loves the Duke. I can't find any humorous moments that spring from this expression of feeling. Viola seems to be really and truly in love. Extra credence is given her feelings here, I think, because she has the advantage of seeing the "real" Duke, as she is in his home undercover as Cesario and not as her real self, Viola. She is privy to his inner most thoughts and feelings without him "putting on" a self because she is a lady. This relationship demonstrates both the heartbreak and triumph of true love, since Viola believes her feelings will always be unrequited, but, in the end, she will marry the man she loves, Duke Orsino.
The Duke is infatuated with Olivia. This is an interesting set-up, as, especially in Act I, scene i, the comedy comes from the extremity of what the Duke believes is the heartbreak of true love. He goes on and on in an overly dramatic tone in this scene, using all the cliches Shakespeare can give him about how deep and true and intense his feelings are. Yet, he is hardly ever onstage with Olivia and seems to love the idea of her, rather than the person herself. I agree that he is infatuated with her as you describe, and this allows his behavior regarding his love for her to be seen by the audience as comic, no matter how tragic and heartbroken he believes himself to be.
This is quite a large question to consider, as the question of Love in Shakespeare's comedies always is. Please consult the Twelfth Night Study Guide for more on these characters and the theme of Love in the play.