I would definitely argue against the suggestion Santana was just being nice because she was getting something. Last week she propped Rachel back up in exactly the bitchy way that would work, and she wasn’t getting anything, and she was being nice, even if it’s a very Santana form of nice.
This week she listened to Mercedes, and then Mercedes chose to bring her in - she didn’t force her way. The opposite, in fact, she strongly encouraged Mercedes to stop trying to get her on the single when it was clear the record producer guy wasn’t into it. She could have tried to manipulate or encourage there and she didn’t.
The Funny Girl thing? Is very slightly different, as she did get to go on as the lead on Broadway, and I would hope she got paid. So yes, she got something out of it, but she also could easily have let Rachel fall flat on her face in her lie. The conversation with Rachel makes it clear she is trying to break out of her old patterns, and she didn’t try to push for what she could get.
It also makes a lot of sense to me that Mercedes would trust her with this, of all people. Mercedes and Santana spent their senior year very effectively working together and sharing the spotlight. When Mercedes was made the leader Santana accepted it, and treated her with respect. The interpersonal drama for the Troubletones was pretty damn low. They made their shared opportunities there, it was always shared.
Obviously show choir is different to an actual record contract, but Mercedes definitely has an experience of working with Santana in a healthy way. She knows that they make each other better, and she really wants to be the best that she can be, more than wanting all the success for herself. Plus, if she’s telling ‘her story’? It involves her friends. That’s just who Mercedes is.