You have just literally said that Sage doesn't think MC could resist Zoey pushing his buttons to steal him from Sage. How on earth does that somehow mean that Sage doesn't doubt in MC and the strength of his feelings for her?
This is not "fear of unknown". It's a negative prediction of the outcome.
Sage being damaged and insecure person thanks to her past experiences is very much part of her character. There's no need to put her on pedestal and pretend those aspects of hers aren't there, or that these intended faults aren't faults. When i think the very point of her character arc is that she'll eventually learn to get over them.
I know her flaws very well. This is a boiling temper like Tommy, a closed nature that is expressed in an unwillingness to talk about feelings, this is a fear of being abandoned. This is her obsession with someone or an idea, because of which she may not notice what is happening around her, she can become obsessed to the unhealthy. This is also a superficial judgment of people (Jill).
Sage grows as a person during the game and overcomes her problems and this is especially good together with MC.
But talking about her as an insecure and complexed girl who thinks that Tremolo will cheat on her is made up. This is directly written in the dialogues between them - Sage does not trust Zoey, she trusts Tremolo, but she doesn't like the idea that Zoey can use her tricks. Tremolo will not cheat on her, but it is still unpleasant and makes her uncomfortable. He is in the company of his ex, Sage knows nothing about and it is unknown what the blue one will do. Plus Tremolo handle it like a retard from the beginning.
And yes, Zoey confesses her love to him, and a more sassy version of her kisses Sage's boyfriend. Fears were justified, I think. Putting myself in Sage's shoes, I would really like to break that expensive stylish manicure on her face.
Don't think I'm blindly protecting Sage from criticism, I have my own views on ex-situations and I follow them.
Cinderella near the top, you love to see it 
