Things I can't determine is a technique to achieve the current category id passed towards the header.php whenever a publish is clicked on.

So let us say I am on category 2 and that i click a publish that goes to multiple groups. I would like the only.perl to keep the look of standing on category 2. To be able to do that I'd require a variable to inform the header which category it originated from

Simply title your template category-X.php, where X may be the specific category title or id you'll need a custom template for, and put it inside a /single folder inside your primary theme directory. Now when a single publish is known as also it matches a current template for the reason that folder, it'll use that to show the publish rather than the standard single.php. If no match is located then single.php can be used.

in functions.php:

define(SINGLE_PATH, TEMPLATEPATH . '/single');
add_filter('single_template', 'force_cat2single_template');  

function force_cat2single_template($single) {
global $wp_query, $post;
foreach((array)get_the_category() as $cat) :

        if(file_exists(SINGLE_PATH . '/single-cat-' . $cat->slug . '.php'))
            return SINGLE_PATH . '/single-cat-' . $cat->slug . '.php';

        elseif(file_exists(SINGLE_PATH . '/single-cat-' . $cat->term_id . '.php'))
            return SINGLE_PATH . '/single-cat-' . $cat->term_id . '.php';
        endforeach;

        if(file_exists(SINGLE_PATH . '/single.php'))
            return SINGLE_PATH . '/single.php';
        elseif(file_exists(SINGLE_PATH . '/default.php'))
            return SINGLE_PATH . '/default.php';
    return $single;
}