As far as I'm aware the only way to get what you wanted would be to NOT select a default category for the front end. This will mean the additional categories will not be restricted to the one you selected. I also figured out how to change the multi select so it's the same as the back end. Just add the following to the top of the page:
JHtml::_('behavior.multiselect');
JHtml::_('formbehavior.chosen', 'select');
