>there is no way to add new Category filter. Clicking on Add button brings up Category search form with disabled search box

When you click Add, you obtain a form. That form includes a pick list control to select the category. Simply click on the pick list, search/select the category, and then Save on that form.

>On unrelated note, New message form ignores ID of a person message directed to

The concept of using the user primary key in the product is disappearing. So, that field should be read only and the pick list can be used to search/select the user.