my dash board leaves a wide a wide space at the middle, (see the screenshot)
I see some component placed between charts and map, so you need to extend either charts box or map / mapBox to avoid blank space. You can learn more about layout rules from our UI Development video (starting from 23:20)
For example how to display information for serial charts from a database, depending on the rights assigned to a particular user
In order to restrict user access to entity instances you can use Access Groups and Constraints. The sample-saas demo app demonstrates an approach to the creation of a multi-tenant CUBA application using Access Groups.
I also need to make the button for start, point to home instead of taking me to a new window
Could you explain in more details what you want to achieve?