Hi,
To change the text, you have to go in HikaShop > Configuration > Languages, edit the language file find the text "login or create a new account" copy/paste the line in the override section and translate the text between the quotes.
www.hikashop.com/en/download/languages.html
Where are these "register" and "not allowed to register" displayed ? I don't recommend to edit the property of .input because it will change many input, it's better to use the parent class name and the input for the property to edit, like: ".parentClass .input{display:none;}
Can you explain with more details what you need ? You just want to display the product images in the checkout ?
If yes, you can edit the view "product / show_default" and remove/comment the part where the images are displayed, and do it too in the view "product / listing_img_title"