The good basic technique to go from specifics to more general context is to look at the calls you already have.
You already call single_cat_title(), which somehow knows current category, right? If you look inside you will see it passes call to single_term_title(). And how does that one in turn knows?
$term = get_queried_object();
So there is your way to get to the full category object when on category archive page.
Still it’s a little unclear from your code example what do you want to do with “all” posts. If you want to display all posts, in a context that already does appropriate query, you should be properly modifying main query via pre_get_post or such. It shouldn’t be done in template for performance and compatibility reasons.