... 5 mins. You will need to setup Joomla and phpBB yourself. We can't help with post to form, you should find the solution yourself. Please also note that where are possible conflicts if using bridge and WP integration for Joomla.

... clone database of any existing subsite. Please create a ticket in our Support Center and attach this template, we will check how it works. Please note that changes to index and CSS files of template will be used by all sites. But MightySites has an option to load custom CSS files for any site, ...

Yes, you will be able to share database contents.
But note that domains won't have same files, so i.e. image from site A won't be loaded in site B, you will need to manually copy article image for all sites.

MightySites is not Joomfish-compatible because it uses own database layer.

Please also note that MightySites doesn't copy content, it shares content: eg. site A loads K2 articles from database of site B.

... articles and their unique content as K2 items? Yes, that's possible, Joomla categories and articles will be same, but K2 data will be own. But note that without proper permissions client can eg. delete article and it will be deleted in all sites. Would that work to my goal? Note that you will ...

"Full site" backup of Akeeba backups both database and files, so each full site backup will restore files used by both sites and database of chosen site.
So Akeeba backup will restore the backup of site where Akeeba is installed, note that files are same for all sites.

All phpBB information will be preserved and you will be able to copy users from phpBB to Joomla. But note that phpBB and Joomla have different password storage methods, so direct login in Joomla with phpBB password will be impossible without a bridge. With bridge installed ...

... create new sub-domains. (customers) Yes, Joomla super admin user in backend of main site site.com will be able to create new sites in MightySites (note that you will need to create a parked domain custx.site.com in your hosting panel first). Admin will also be able to auto-login to backend panels ...

... Joomla installation is better for you, in this case all files including images will be also shared, so you won't have problems with avatars. Note that MightySites shares database data only, not images. Using different installation will create additional problems, eg. user uploads avatar in ...

You can share only ALL menus and menu items, same for article - only ALL articles. But note that each site can be configured to display only its articles: so all sites will load same categories and articles, but each site will display articles only from selected category. ...