1) Sorry but I don't know this administration tool.
When you want to create a slave site with domain or subdomain, you have to specify in your administration tool the location on the disk (root directory).
So both JMS and you administration tool must have the same value.
If you don't know the value to enter, once you have setup the domain in your admin tool, you can put the "hello.php" file (see tutorial video 7) to know the current path.
2) If you want a specific training for this hosting administration, I suggest that you order our billable support and 1 hour of training.
www.jms2win.com/en/joomla-multisite?page...35&category_id=1
The sharing of the content of an extension is illustrated in the tutorial video 16 and the creation of a slave site with cPanel is illustrated in tutorial video CP-01
www.jms2win.com/en/tutorial#jms12x-16
www.jms2win.com/en/tutorial#jms12x-CP-01
The concept is identical except that in your administration tool, the cPanel "document root" is called "Root directory"
Concerning the Symbolic Link, perhaps that you will have to use a specific PHP mode.
Perhaps that you would have to use the "FastCGI" (to test).