back to parent category

um, isn’t that what breadcrumbs are for?

get_category just prints out link(s) to the applicable categories. You need to use get_terms to access the taxonomy objects and find the ones with no parent

$terms = get_the_terms($post->ID,'product_cat');

$count = count($terms); $i=0;
if ($count > 0) {
    $term_list = '<span class="parent_category">Parent Category:';
    foreach ($terms as $term) {
        $i++;
if($term->parent==0){
    	$term_list .= '<a href="/product-category/' . $term->slug . '" title="' . sprintf(__('View all post filed under %s', 'my_localization_domain'), $term->name) . '">' . $term->name . '</a>';
    	if ($count != $i) $term_list .= ' &middot; '; }
    }
    echo $term_list .'</span>';