Hi Patrice,
1. No difference. They offer the same functionality, implemented in different way, with slightly different options. The "Fixed Header" atom is simpler, while the
UIkit Sticky component is more advanced and offer more options.
2. With all options ("Headroom.js, "Fixed Header" and "UIkit Sticky") a CSS class is being added to the element when it gets fixed/sticky. You can use this class and write your custom CSS code to adjust the Header, the Logo, whatever, when the Header gets fixed/sticky.
But this is something you need to do on your own. It is out of the support scope.
You can have a look at our Milano (uses UIkit Sticky) and Paradise (uses the "Fixed Header" atom) templates and see what we have done there.
3. Yes, exactly. These are the only differences.
4. Nope, that's not possible.