Hi,
1. A module class suffix is to be added to a module in the modules manager of Joomla when you edit a module.
The product page is not a module. It is displayed by your template in the main area of your template, so you can't add a module class suffix to it.
What you can do however is to add the same CSS that you added for the class well to the class hikashop_product_page and then you'll get the same thing displayed. You can add such CSS in your Joomla template or in the HikaShop frontend CSS as explained in the link Xavier gave you earlier.
2. There is no need to edit any product view file for that. Again, that's something you can do with CSS that you can add on your website. Probably something around those lines:
#hikashop_product_left_part{ width :100% !important; }
3. That's probably coming from the CSS of your template as we don't have the problem on our end. Unfortunately, the link that you had provided in your first message doesn't work anymore so I couldn't look at the issue on your website.