when john high-fives Kernelsprite and later Nannasprite in acts 1-3, why doesn't he fuse with them?

He’s a core player of the game. Prototyping him would create a Doomed Timeline immediately. People also tend to forget Seppucrowsprite pecked Dave’s head and brought a lot of items to make a nest. Maybe you can only prototype a Kernel if it falls right into the core, and then fully formed sprites are more lenient in that regard?

They also favor dead/doomed things, so a perfectly undoomed boy could be straight up rejected for Prototyping by Skaia’s Standards.