I am trying to understand 2 things.

If I have a master with extensions, and lets say 10 slaves all using the extensions and everything is working. What is the procedure for adding a new extensions later? I assume I add it to the master but then do I edit each slave to have sharing capabilities?

How do you backup sites with multiple slave using different subdomains? Will a cpanel full site backup also backup all subdomains?

Thanks in Advance.

When you add an extension, you effectivelly needs to first install the extension in the master and after re-install the extension in the slave site.

If you have created an architecture like presented in the tutorial video 0.b slide 21-24, you have created a "group" slave site that can be used to propagate a sharing action to a set of slave site.
When you have a large number of slave sites, it might be interesting to have all the website grouped together to allow using the JMS tool menu where you have the possibility to propagate the current action (install, uninstall, sharing) to all the children websites. So you could do it for all the websites in very few steps.

If you apply the recommendation present in the tutorial , you may have to do maximum 3 steps:
1) Install in the master
2) Install in the "main" (root) slave site
3) Share the extension between the group and the main(root) site with propagation to all the children.

If never need to share anything, you can do it in 2 steps because you could group all the website directly on he main (root) slave site and directly propagate the installation to all the children.

Concerning the backup, see the FAQ where you present the manual procedure and where we also provide a shell script to perform a weekly backup. The shell script can be used in a cron in cPanel.
www.jms2win.com/en/faq/faq-joomla-multi-sites#cat-268
www.jms2win.com/en/component/docman/doc_...weekly-backup-script

You can also modify the shell script to perform a backup of all the DBs in one shot.