I figured it out and got it to work right, I had to add more else if statements to your example code, and will need to do this everywhere different files are called upon like for for example the product info display files for different devices. If there is a way to start the session when it detects a device then this extra code wouldn't be needed. I also had to do this to the code in the html header file to load the correct stylesheets so my headers would look right.
if ($detect->isMobile() && !$detect->isTablet() && $_SESSION['display_mode']=='isMobile' or $detect->isTablet() && $_SESSION['display_mode']=='isMobile' or $_SESSION['display_mode']=='isMobile'){
require($template->get_template_dir('tpl_header_mobile.php',DIR_WS_TEMPLATE, $current_page_base,'common'). '/tpl_header_mobile.php');
} else if ($detect->isMobile() && $_SESSION['display_mode']=='isTablet' or $detect->isTablet() && $_SESSION['display_mode']=='isTablet' or $_SESSION['display_mode']=='isTablet'){
require($template->get_template_dir('tpl_header_tablet.php',DIR_WS_TEMPLATE, $current_page_base,'common'). '/tpl_header_tablet.php');
} else if ($_SESSION['display_mode']=='isDesktop'){
require($template->get_template_dir('tpl_header.php',DIR_WS_TEMPLATE, $current_page_base,'common'). '/tpl_header.php');
} else if ($detect->isMobile()){
require($template->get_template_dir('tpl_header_mobile.php',DIR_WS_TEMPLATE, $current_page_base,'common'). '/tpl_header_mobile.php');
} else if ($detect->isTablet()){
require($template->get_template_dir('tpl_header_tablet.php',DIR_WS_TEMPLATE, $current_page_base,'common'). '/tpl_header_tablet.php');
} else {
require($template->get_template_dir('tpl_header.php',DIR_WS_TEMPLATE, $current_page_base,'common'). '/tpl_header.php');
}