Tableau PostgreSQL DB - custom reporting for Site Limits and users

We do not have Admin access to our Tableau Server, but we have access to PostgreSQL DB workgroup schema. 

We have a need to monitor LDAP groups, users and Site Limits. 

We have created custom data model for the data (screenshot attached), but results do not match with details presented on Tableau Server (supporting group with Admin access is sharing latest Site Limits numbers) and some of the users do not have names displayed. 

Please let me know if our model is using proper objects. 

Is there any documentation for the workgroup schema tables how to connect them? 

We start with users: 

- id 

- site_id 

- site_role_id 

joined group_users via

= group_users.user_id for: 

- group_id 

joined groups via group_users.group_id =

for: 

- name (group name) 

joined hist_users via

= hist_users.user_id for: 

- name (user name) 

joined site_roles via users.site_role_id = site_roles.id for: 

- display_name (site role name) 

joined sites via users.site_id =

for:  

- name (site name) 

Viz 1 

Rows 

site_role_id, display_name, countdistinct (user_id) 

we have number of users for each site role. We can add here role_name, we can add user_name, but there are NULL values for user_name from hist_users.  Why there are NULL for user_name? Are they past users? Never logged in? 

Viz 2 

We would like to find number of Site Limits for each license type. 

In sites table there are three columns named 'Tier Basic User Capacity', 'Tier Author Capacity', 'Tier Interactor Capacity'. Are them the ones to preset set Site Limits? If no, where can I found proper numbers?