Reordering entries tied to categories?
If you reverse the relationship then you should be able to order the entries (i.e. add products to an entries field within the category, instead of adding categories to a categories field within the product).

So instead of grabbing the products using relatedTo(category),

{% set products = craft.entries.section('products').relatedTo(category) %}
{% for product in products %}...{% endfor %}


you would be able to grab the products directly from the products entries field in the category using category.productsFieldHandle, which should be returned in the order defined in the CP.

{% set products = category.productsEntriesFieldHandle %}
{% for product in products %}...{% endfor %}