Why the character who knows the answer belongs early
Your story is a refusal that ends rather than a search that succeeds, so the character who already lives the answer belongs on the page early.
How to write a mentor who wants something
A mentor's persistence is free once he needs it to work, and his authority comes from having survived the weakness your protagonist has now rather than from seniority.
A mentor who wants nothing is just someone handing out good lines.
One of StoryDNA's fundamental characters is the Mentor: someone who teaches your protagonist what they learned the hard way. Give your Mentor their own reason to need it to work, and you never have to explain why your Mentor keeps coming back after being told to leave.
In Rocky, Mickey Goldmill isn't just a great trainer passing on skills. He's a man who never got his own shot, watching one walk past him.
A mentor's authority doesn't come from age or rank. It comes from having carried the same weakness your protagonist carries now. Nothing else holds up when your protagonist is really hurting.
That's why Dory works as a mentor in Finding Nemo. She has no standing and no wisdom she can put into words. But her refusal to stop swimming is the one thing Marlin's fear can't argue with.
The people who help you often need it to work more than you do, because your recovery tells them their own meant something.
Your Mentor should stand closer than your protagonist to what your story argues for. Closer, not all the way there. If your Mentor has fully arrived, you've written a different character.
So write down what your Mentor loses if your protagonist fails. A daughter they don't speak to. Knowledge that dies with them. Then your Mentor will keep showing up without being asked.
Why theme lives between characters, not inside one
What a reader experiences is not a character but the space between two of them, so the same claim carried by three different pairs is what stops a theme sounding like one note.
Why one right choice at the end doesn't prove your theme
A protagonist's climactic choice on its own is consistent with your story's position without ever establishing it, because a second position would explain the same choice equally well.