Hi,
There is no option to display the custom product fields in the cart edit form of the backend.
We thought it would be unnecessary.
Regarding your error with selection of the product, it comes from a javascript error when the popup is being loaded.
To fix it, add the code:
<div id="header"><div class="header-title"></div><div class="header-items"></div></div>
<div id="header-more-items"></div>
at the beginning of the file administrator/components/com_hikashop/views/product/tmpl/selection.php and it will remove the javascript error which will allow the javascript to be properly processed.