melichades

solved partially:

created an second page, containing a list of full-width, one-column “thumbnails”, cropped to the full width of the page e.g. 1024px:
crop: width: 1024px height: 0

altered the quality-field in includes/global/common.php / this is a real crude solution 🙁
if ($retina) $quality=15; else $quality=30;
(how could i do this in my own functions.php?)

inserted a function in my functions.php that changes all links to the permalink of the details-page:

add_filter( 'envira_gallery_pre_data', 'tgm_envira_disable_lightbox', 10, 2 );
function tgm_envira_disable_lightbox( $data, $gallery_id ) {
	// You can target this by specific ID if you want. If you want this for all galleries, just remove this line. Change 324 to your gallery ID.
	// if ( 324 !== $gallery_id ) {	return $data; }

	// Change the link field for each image.
	foreach ( (array) $data['gallery'] as $id => $item ) {
		if ( ! empty( $data['gallery'][$id]['link'] ) ) {
			// only, if no page is targeted:
                        if (substr(strtolower($data['gallery'][$id]['link']),-4,4)=='.jpg') {
				$data['gallery'][$id]['link']=get_page_link();
				$data['gallery'][$id]['link']=preg_replace('/\/$/', '_detail/', $data['gallery'][$id]['link'] );
				// link back to preview-page
                                $data['gallery'][$id]['link']=preg_replace('/_detail_detail\/$/', '/', $data['gallery'][$id]['link'] );
			}
		}
		// fill the alt-tag
                $data['gallery'][$id]['alt']=$data['gallery'][$id]['caption'];
		// fill title tag
                $data['gallery'][$id]['title']=$data['gallery'][$id]['caption'];
	}
	return $data;
}