Do you need to share files (so slave sites will be parked domains and all sites will share same server folder, hence use same files) or not (each slave subdomain will use own subfolder and have own Joomla files)? Our case studies cover all possible ...

Yes, exactly, MightySites allows to run multiple Joomla websites from single installation of Joomla on master site.

Slave sites are created as parked domains in hosting panel - so they use same files.
But each site can use own database tables and hence have own contents.

Yes, it's possible to have slave sites like http://domain.com
Such domain name can also be created as parked domain (domain which shares server folder with master site and hence use same files).
Please ask your hosting provider about such option first.

Yes, but first you need to decide if subdomain will use own files or files of main site (in this case it should be created as parked domain).

... site. MySQL users of slave sites should have access to database of main site. Each site can either use own files or use same files for all sites (parked domains). 1) Does each site have its own admin panel to edit menus, forums, etc for that site only? Yes, each site has own admin panel as usual. ...

You need to use relative paths, in this case it will be fine.

Probably you don't understand: either parked domain xxx.site.com or symlink site.com/xxx means that this virtual site uses same files as main site. So you can add/edit/delete files in virtual site and main site will also get these changes.

Yes, you can create a virtual site like xxx.mainsite.com (this domain first should be created as parked domain in your hosting panel) or mainsite.com/xxx (no need to create domain). There is an ability to clone database tables of current site for this new site. Next you ...

Your server provider should setup certificates, it's not easy: each parked domain should be recreated as separate virtual host in apache config.

Did you figure out how to setup seperate SSL certificates for the parked domains? Was it possible with your setup?

... configuration: 1. Run all these sites from single hosting account of main site, so all sites will use same files. Slave sites should be created as parked domains in your hosting panel of main site. 2. All sites will use same database tables, so ALL contents will be same. So no need to copy database ...