How can I add an extra button & submit handler to an exposed view form?
The submit handler is not called by default on a form api element type "button". If you want to execute a submit handler on a button you will need to add

$form['my_button']['#executes_submit_callback'] = TRUE;


As a side note this will not be the case if the #type was 'submit' because the executes_submit_callback is by default equal to true on that element type.

Hope this helps.