2 & 3) There are two parts to these questions: A) assigning rights to users based on sites and B) figuring out which sites contain the content you are interested in.
For part A check out “Understanding Operator Rights” in the
Admin Guide
. Generally, you are going to go to the Operator Permissions tab in a site and assign access to specific Operators. Part B is a little more complicated because you have to know which sites contain the content you are interested in controlling. For Fixlets, Tasks, and Analyses this is easy, you just look at the Site column in the list panel or look at their folders in the tree view. For Wizards and Dashboards you can go to the All Content domain and look in the tree view, the Wizards and Dashboards will be in folders named for the Sites that they originate from.
If you are talking about the Client UI, then this will help:
If you are talking about the Console then the answer is “custom dashboard development not available to customers, only folks with access to internal development tools can do that.”