is this Nintex Forms 2010 you're using?
You can't simulate mobile view in your desktop browser as classic Nintex Forms aren't responsive. Closest point to come to is creating a Smartphone Layout in the Layouts pane (screenshot is from 2013 but the function should be the same)
Did you already do that?
I'm not sure if you can hide the top area as it belongs to SharePoint. But did you consider using the Nintex Mobile App? It let's you create dedicated mobile forms and also won't show the top area. Also it's free. Maybe you can give it a shot.
In classic forms you won't have a dedicated landscape mode as the forms are not responsive.
But if you're using SharePoint 2013 I suggest you update to the latest Nintex Forms version which brings responsive forms. It's also applicable to SharePoint 2013 Standard edition. I expect that responsive forms behave much more like you expect it.
The free Nintex Mobile App needs to be downloaded by each user. The enterprise edition however can be distributed to your colleagues by Mobile Device Management (MDM) automatically.
I'm not sure if I understood your last question. In general you can insert any CSS or JS includes but you can't replace the whole overall form. However you can create a Nintex Live form which will be held in Nintex cloud rather than in SharePoint which allows you to insert the plain form without any other stuff into your webpage or whereever.