This is pulling the content under the navigation.
I am not sure why you need the merged header element as that is only really used when you want to have a transparent site header overlaying the content.
You can find the in Appearance > Elements. You can quick edit it and set the Status to draft to disable it, or simply delete.
2. Your pages have a Content Title set in the WP post editor enabled, its the bottom margin attached to the heading that is creating the white gap. You will see the title when you remove the merged header element.
You can remove the tile using the Disable Elements meta box in the WP editor: