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The letter shows in a nutshell the girl's had no true meaning to her :\ otherwise why take something that was never related to her and retcon it to be related to her. Jeez.
Anyhows, 'sorrows to end' can mean Kairi, Sora, and let's pretend Riku as well. They've been through enough to make it work without needing conspiracy theories to link it to coded.
Nomura might have planned that from the jump.