When you say "if head-first and complement-first were robust, universal features of languages", do you mean "head-first and complement-first cover the whole set of possibilities for the property 'word order'", or "all languages (within a family?) are one or the other", or something more like "a given language cannot contain both head-first and complement-first structures", or something different?
Also, with "appearances of word orders demonstrating these features are dependent on each other across language families", in what sense do you mean that the appearances would be dependent on each other?