Hi,
1. The "backend listing" setting of your custom item field is turned off, so it's normal that it doesn't appear on the listing of the products of the order in the backend.
2. You can add custom CSS to the backend CSS of HikaShop via the HikaShop configuration.
3. a In the file administrator/components/com_hikashop/views/order/view.html.php, you'll find that code:
array('name' => 'Popup', 'icon' => 'send', 'id' => 'send', 'alt' => JText::_('HIKA_EMAIL'), 'url' => $url_email, 'width' => 720),
array('name' => 'Popup', 'icon' => 'invoice', 'id' => 'invoice', 'alt' => JText::_('INVOICE'), 'url' => $url_invoice, 'width' => 720),
array('name' => 'Popup', 'icon' => 'shipping', 'id' => 'shipping', 'alt' => JText::_('SHIPPING_INVOICE'), 'url' => $url_shipping, 'width' => 720),
Change the width in order to control the size of the popups.
3.b Go in the menu System>Emails, click on the email you want to modify and do the change in the HTML version.
3.c It can come from several things.
First, download the install package of the latest version on our website and install it on your website. Then, save the settings of your custom field. If that still doesn't display the custom item field, check that you don't have any overrides of the emails via the menu System>Emails. If you do, try deleting them.