If have posts a week old, display, else display a different loop

by

The code you have provided already has a check to see if posts exist for said query. So all you need to do is expand on it.

...
    <?php else: ?>
        <p>There is no news related to this theme</p>

    <?php endif; ?>
...

Simply place you next query posts in the above code replacing <p>There is no news related to this theme</p>