Welcome 
the answer to your questions
- the sonarqube maven plugin is just a wrapper with sane defaults taken from pom.xml
- it has to download the scanner plugins, i.e. Sonar Java … etc. once
- Other !? don’t get it !?
I don’t quite understand your setting / environment !?
We have several network areas, but use only one Sonarqube Enterprise instance for all.
One principle of Sonarqube is that developers are responsible for the quality of their software.
So they should / must have access to their results and metrics in Sonarqube.
Gilbert