none value should not be available as deepest term

Sorry, but this won't work.
I've tried to implement this but it causes more problems (for example setting an initial value in the next level would require to create the next-but-one level also but this won't happen until the user selects a value).
Leaving - None - as an option would force the user to select a value from the dropdown and ensures all preceding js magic is executed correctly.

Hiding the option is not the problem (from a technical perspective). The main problem here is that the user needs to manually select an option to make all the background-functions do their work (for example "display dropdown for the next level").
If I remove - None - the user action needs to be triggered automatically for the first option and this leads to more problems ...