New questions, belong in new threads. Unless it is just an extension of the original question.
In answer to your PSS: You can modify multiple products at once, try doing this and adding them to category.
I don't believe that hiding an active category is possible without modification of smarty or PHP code. I would start by adding the option to the category page in admin, a simple checkbox for "hide". Then you can work on the front side, taking action based on that checkbox.
Adding a custom variable to a category is relatively simple to do. I have a tutorial that shows how to do it for a product:
https://www.youtube.com/watch?v=-7ESOW0pKU4
Adapting what you learn there, the same technique can easily be applied to a category.