It is very hard to test/debug this kind of question since it is dependent upon a lot of code but it looks to me like you just need to break apart your comma separated list, get the category data, and print the names. get_categories()
and wp_list_pluck()
should make that pretty easy.
//Display category column
function column_post_category($item) {
$c = get_categories(array('include' => $item['post_category']));
$c = wp_list_pluck($c,'name');
$c = implode(', ',$c);
return $c;
}