In answer to your questions:
1. Currently yes there is an existing Spanish language file available for Quality control
here.
2. There is no custom form builder in the theme, so it would require some additional customization on your part if you were looking to add other fields.
3. Yes
4. Quality Control is a stand alone theme, so it's not designed to be integrated into an existing theme in the way that a plugin is. You can of course restyle Quality Control if you decide to use it on an existing site (perhaps in a subdirectory), so that it matches your existing homepage. Thanks.