In Opencart 2.1 it doesn't show logs, duplicate orders and other errors. How to fix?

Question

opencart

Hello.
There is a site on Opencart
Already after accepting the work, they found errors, but the developer does not respond from a certain point and does not want to fix the errors.
I understand that, perhaps, without a programmer it will not be possible to fix it, but perhaps you can at least tell me the direction.
I have knowledge of HTML and CSS, and I also know a little about the Opencart admin panel and made minor edits to tpl files.
List of errors.
1) Error log - Access denied! Also the file /system/logs/error.log is empty.
In user groups, all rights for the user are set, but it did not help. I read that it could be due to the incompatibility of some modules - then how to determine what the problem is?
2) Sometimes there is a duplication of the order. For example, user Olga ordered item1 in order 547, and then after some time ordered item2 and item3 in order 570, and item1 from the previous order is added to the order. And it doesn't always happen. Some users have many different orders and they are not duplicated.
3) On the page of any product in the "Examples of the product" when you click on the photo, a popup gallery appears and regardless of which picture you click on the parent page in the popup, everything starts from the first photo. As I understand it, this is magnific-popup and you need to look at its settings?
5) On the default product page in the accordion, reviews are opened, not a description. And you need just the opposite. I found the accordion.tpl template and in it
in the description paragraph

<a data-toggle="collapse" data-parent="#accordion-v5" href="#collapse-v5-description">
    <?php echo $tab_description; ?>
</a>

and in paragraph reviews

<a data-toggle="collapse" data-parent="#accordion-v5" class="collapsed" href="#collapse-v5-tab_review">
    Отзывы
</a>

That is, the block with reviews is collapsed, and when the site loads, the description is collapsed.
Maybe it's correct elsewhere?
6) On any page with a product in the "You recently viewed" block, if you click on the image, then the error "The image could not be loaded", but if you click on the name of the product, it normally goes to the product.
I understand that there are too many questions, but maybe at least for some reason you will tell where to look. 

Answer 1

1 - system users, user groups.
3 - Layout flew, tpl look
a little later I will give more answers.