No, this function is no longer supported as of Friendica 3.3 onwards.
You can find the main repository here. There you will always find the current stable version of friendica.
Addons are listed at this page.
If you are searching for new themes, you can find them at github.com/bkil/friendica-themes
Have a look into your config/local.config.php and fix your email address there.
Yes. You just have to list more then one email address in the config/local.config.php file. The listed emails need to be separated by a comma like this:
'admin_email' => 'firstname.lastname@example.org,email@example.com',
Please have a look at the Admin panel under DB updates and follow the link to check database structure. This will start a background process to check if the structure is up to the current definition.
You can manually execute the structure update from the CLI in the base directory of your Friendica installation by running the following command:
bin/console dbstructure update
if there occur any errors, please contact the support forum.