Hi,
2. this comes either from the HikaShop translation file that you're using, or directly from your template.
So check your translation file via the Languages tab of the configuration and if you don't find that text there, it means that it's the template which does that.
4. The emails can be edited via the menu System>Emails with the Business edition. With the other editions you would have to edit the files directly in the folder media/com_hikashop/mail but please note that you'll loose your changes after an update of HikaShop if you don't do it via the interface of the Business edition, so make sure that you do a backup.
5. You would have to edit the file "cart" of the view "checkout" via the menu Display>Views and add such code:
<?php
$delete = hikashop_completeLink('product&task=cleancart');
if(strpos($delete,'?')){
$delete.='&';
}else{
$delete.='?';
} ?>
<a class="hikashop_small_cart_clean_link" onclick="window.location='<?php echo $delete.'return_url='; ?>'+window.btoa(window.location); return false;" href="#" >
Clear cart
</a>