Adding an option to a shortcode

Regarding Edit #3 I would rewrite it like this:

//* Shortcode for getting users
function list_of_users( $atts ) {

    extract( shortcode_atts(
        array(
            'display' => 'all',
            'user' => '30'
        ),
        $atts
    ));

    $content="";
    switch ( $display ) {
        case 'all':
            $content = display_all_users();
            break;
        case 'single':
            $content = display_single_user( (int) $user );
            break;
        default:
        break;
    }
    return $content;
}
add_shortcode('staff', 'list_of_users');

function display_all_users(){ 
    $args = array(
        'orderby' => 'ID',
        'order' => 'ASC'
    );
    $users = get_users( $args );

    $html="<ul class="staff">";
        foreach( $users as $user ){ 
            $user_info = get_userdata($user->ID);
            $html .= '<li>';
            $html .= '<a href="'.get_home_url().'/author/'.$user_info->user_nicename.'" class="staff-image">';
            $html .= mt_profile_img( $user->ID, array('size' => '250x250', 'echo' => false));
            $html .= '</a>';
            $html .= '<div class="staff-info"><a href="'.get_home_url().'/author/'.$user_info->user_nicename.'" class="staff-name"><h2>'.$user->display_name.'</h2></a>';
            $html .= '<div class="service-certs">';
            $html .= get_field('certifications','user_'.$user->ID);
            $html .= '</div>';
            $html .= '<p class="service-excerpt">';
            $html .= get_field('short_bio','user_'.$user->ID);
            $html .= '</p>';
            $html .= '<a href="'.get_home_url().'/author/'.$user_info->user_nicename.'" class="more-staff-bio">Read more from ' . $user_info->user_firstname . '</a></div><div style="clear: both;"></div>';
            $html .= '</li>';
        }
    $html .= '</ul>';
    return $html;
}

function display_single_user( $user_id = 30 ){

    $html="<div class="home-profile-image">";
    $html .=  mt_profile_img( $user_id, array('size' => '175x175', 'echo' => false));
    $html .= '</div><div class="home-short-bio">';
    $html .=  get_field('short_bio','user_'.$user_id);
    $html .= '</div><a href="'.get_home_url().'/our-team/" class="button">Read more staff bios</a>';

    return $html;
}