Your protagonist is the character who grows the most over the course of your story. While they are generally the same character as the main character, they don’t need to be. This might be a bit confusing to write at the beginning, but it is not completely unheard of. I would argue that in the Harry Potter series, Harry Potter is the main character while Neville Longbottom is the protagonist. I admit, it’s a bit of a stretch, but it gives a clear example of what I’m trying to describe.
Your plot can be anything you wish, to be quite honest. Having two different characters for the protagonist and MC will not make or break different plot types. The protagonist could be the main character’s enemy (making your main character an antagonist), or someone the main characters mentors to grow– the possibilities are endless. Just make sure you give the necessary amount of time to your protagonist, because if she isn’t growing, then she isn’t a protagonist.
For something like this, I would recommend against using multiple points of view when writing, because then your protagonist will become a main character, and then all of the hard work you’ve put in to build up a protagonist without her being a main character will be for nought.
I wish I could be more helpful, but it isn’t all that different from writing a typical MC/protagonist combo. Writing with this cast structure is a really good exercise for anyone who is looking to test their character development skills. It can be much harder than it sounds. Good luck, and happy writing!