Seems to me you are a bit overconfident that "we" (who is "we"?) understand how the brain works. F.ex. how does a neuron actively stretching a tentacle trying to reach other neurons work in your model? Genuine question, I am not looking to make fun of you, it's just that your confidence seems a bit much.

The simplified answer to that is some sort of a chemical gradient determined by gene expression in the cell. This is pretty much how all biological activity happens, like how limbs "know" to grow in a direction or how butterfly wings "know" to form the shape of a wing. Scientists are continuously uncovering more and more knowledge about various biological processes across life forms and there is nothing here to indicate it is anything but chemical signalling. I'm not a biologist so I won't be able to give explanations n-levels deep, but there is plenty information accessible to form an understanding of these processes in terms of physical and chemical laws. For how neurons connect, you can look up synaptogenesis and start from there.