On mobile the theme switches to “static”-no slider. It was feature image of first the location in ST Slide Location element. You can edit feature image of this location to change image behind the search field on mobile.
You can see my attachment.
We have developed mega menu in theme version 2.0.1. It will available in few day.
Please wait for it.
I don’t understand your membership issue. Please description more details so i can help you better.