Thank you lexxy2000 to allow me sleeping a little and help me answer to all the post.
This is effectivelly help full and welcome.
I see that you have started to get the control of JMS and begin to manage different concepts present in JMS. Congratulation.

To bailey, I will add the following points.
If you want to replicate your master into the "slave_master", you can create a JMS templates rules that will replicate your current master with all the tons of extension pre-installed.

The concept of the JMS Templates is partially presented in the tutorial video 13 that show how to create a the rule to create a website from the front-end.
The same concept can be used for the back-end.

In addition, the reason why we do not recommend to share or use the master website directly is to driven by security issue.
The global security of a website is determined by the less secured extension installed on a website.
We consider Joomla as secured but this is not necessary the case with all the extensions that might be installed on a website.
As you have to install all the extension on the master website before to re-install (perhaps partially) on a slave site, therefore the vulnerability of your master website is less secured than a slave site that can have only few extensions installed.
You must consider the master website like a library or a repository of all the extension that you might need in your slave site.