That 'head not needing to grow bit' is absolutely ridiculous in my opinion...
How many years ago was the theory of Lamarkism de-bunked???
This theory says that, for example, a giraffe has a long neck as they stretch up to eat leaves, and that each subsequent generation stretched up further to eat the leaves higher up the tree... by this arguement, a baby that has had an arm amputated, and learned to get along fine without it (ie never needing to use it) would then pass on this trait and ttheir child would be born with only one arm...ridiculous!
Environment can certainly influence growth in general, but in general only. It is far more likely that as the biggest bones in the snakes body, the bones in the head simply take longer to grow, and cannot respond to an unaturally accelerated growth rate like the body does.
This accelerated growth rate, even as the head catches up in the end, I believe it is unhealthy, placing strain on the animal.