Will an alternative launcher stop Galaxy S4 additions from working?
In general, anything that works regardless of which app you have in the foreground will work regardless of which launcher you have installed. That includes eye scrolling, air gestures, smart stay, smart orientation change (I forget the marketing name for it), and so on.

Changing to a different launcher will only replace the home screen and the "all apps" menu. Note that the old launcher is still installed: the first time you go to the home screen, you get a chooser to pick which one to use, the same way as if you have more than one web browser and click on a link.