This plot reveals Santana's motivation to be equal parts petty jealousy and protector. She wants to make sure that Brittany is happy, but she is no longer the one who is going to make that happen. Were Santana to toss her hat back in the ring, and it is something she considers when Sue (Jane Lynch) offers her a job at the school, she would have reason to be against Sam. But since Santana doesn't intend to permanently return to Ohio, and Sam makes Brittany happy, Santana needs to accept things as they are. This episode doesn't quite take us to that resolution.
"Diva" does deliver Santana to New York, which is a happy development. I don't know that Santana really fits exactly with Kurt and Rachel, but she is a fantastic character whom I have missed, and her inclusion in their circle should shake up the dynamic in a fresh and interesting way. Perhaps she should have asked to move in with them, rather than just declaring that she lives there now, but it's still my favorite scene of the episode.
Sadly, Santana's musical numbers in this installment leave something to be desire. Given her past credits, Santana absolutely belongs in an episode entitled "Diva." Yet, neither "Nutbush City Limits" nor "Make No Mistake, She's Mine" are that enjoyable. This is especially regrettable in the latter, since a Sam-Santana showdown should have been an awesome moment. She does slay "Girl on Fire," but it's not a very good song in the first place, in my opinion, so I didn't find any memorable Santana music moments in "Diva."
Also, if Santana dropped out of college in Kentucky a month ago, why does she show up with Louisville cheerleader back up singers, in uniform, and a girlfriend classmate? Are they still dating? Even if they are, it can't possibly be that serious, given the way it is presented, and certainly not enough to warrant a trip to Ohio, with friends in tow.
The other arc unfolding in "Diva" begins when Finn asks Emma (Jayma Mays) for assistance motivating the students. She lends her ideas, which leads to the diva assignment, and helps set Finn's group back on track.
Finn is doubting himself, but Emma is even more a mess, swamped under wedding planning. How does she have the time to give him a hand? Why does he even ask, when, if he really cares, he should be helping her, not piling on her workload?