For responsive settings, you can use width=”100%” (as you’ve done) and adaptive=”true”. The latter just recenters the map if the screen size changes.
I think the white strip is coming from the theme – some themes apply styles to tables or images. You could try renaming the theme’s style.css and see if it’s still happening.
There are some mashup examples in the MapPress documentation. For two categories I think you’d want to use category__in=a,b. The WP_Query syntax is listed here if it helps: