I think this logic works for everything but interpretations of appendices ('123-Appendices-Interp'). These would skip the first guard (as 'Subpart' isn't present), but hit the second (as 'Interp' is), becoming '123-Interp', no?

Even if it missed the second case and fell into the third (as you've commented), it'd become '123-Appendices', which is also not right.