Is there a way to return a list of the default image size names when they are unset?

Call get_intermediate_image_sizes() before applying the filter (intermediate_image_sizes). Or, temporarily remove the filter, call the function to store the output, and then re-apply the filter.

If you only want the list of default image sizes, then you can remove the values returned by wp_get_additional_image_sizes() from those returned by get_intermediate_image_sizes() (untested):

remove_filter( 'intermediate_image_sizes', ... );

$image_sizes         = get_intermediate_image_sizes();
$addtl_image_sizes   = wp_get_additional_image_sizes();
$default_image_sizes = array_diff( $image_sizes, $addtl_image_sizes );

add_filter( 'intermediate_image_sizes', ... );