What I thought of it is that his ability to survive the horror (Uri Orlev) and find hope was what the ending meant. It's a semi-autobiography so in the end I think he wrote such an ending to signify the hope you were talking about, and Peter's reunion with his father was the own author's reunion with peace, with hope, with the ability to hold on even if the horrors you experience have robbed you of any reason to even keep holding on, his island, his prison, his cage was what kept him down, what kept yelling into him "GIVE UP, LEAVE, RESCUE YOURSELF, THERE'S NOTHING FOR YOU HERE" but his commitment to family, his commitment to love, his commitment to hope is what kept him there, because his father is coming, it was his promise.
But I feel that Uri Orlev's death of his mother was the island, was the screeching scream that was telling him GIVE IN, GIVE UP, SURRENDER TO YOUR MISERY, IT'S ALL YOU HAVE LEFT. But he knows that his mother would want him to continue living, to live a good life, and be good to hold onto hope. So the reunion with his father may seem like a fairy tale ending that just doesn't fit. But I feel that that reunion is an incarnation of his own reunion with peace, the fact he lived through all of that misery and death, that raw madness, to finally see the end of it. The fact he was able to turn his love of literature into a realized career of writing, for children, to bring hope into them, that hope he kept cherished and protected like his mouse, Snow. When Snow dies, his father returns, I think in a way the thing he loved and the thing that brought him comfort such as writing, was what Snow represented, during the war, during the chaos, he kept his love for literature alive, he kept it close to his heart, and when all seems lost, and he won't ever be able to ever see hope again (The death of Snow, not surviving the war and realizing his dreams of writing).
His father comes, peace comes, hope is replenished, and he's able to step off the island, the isolation, the madness, he's able to come forward and be protected, and loved by the very peace and hope he instilled onto Snow.
So the peace he tried to keep alive with his love of books, is the very peace that he's washed over with the end of the war, the end of the suffering, and he's able to now nurture and love his craft, his family, himself. I saw this once late at night when I was 8 or 9 years old, loved it but I never saw it in it's entirety, I finally got back to watching it now. And I really did love it. It's a beautiful movie, fills you with a huge splash of hope