Shared Multisite Settings
so for what it's worth using the default/settings.php as the master and then having each of the sites do a PHP include (as @Alan mentions) does work well!  (I know this is an old question but I am working through multisite issues with Drupal 8 (8.6.13) and have come across this very thing...)

a couple of caveats:

1) I am managing the multisite installation and config manually so am creating my own settings.php files, not using drush site-install which will auto-generate the settings.php

2) you must do the include at the top of the site's settings.php and ensure that you are overriding only what you need to. (...or the inverse, that you are okay using the config in the default settings file)

I have this at the top of the settings.php for each site:

$default_path = "sites/default";
include $app_root . '/' . $default_path . '/settings.php';


and then override the database settings - setting the prefix etc:

$databases['default']['default'] = array (
  'database' =&gt; 'this_is_the_database',
  'username' =&gt; 'username',
  'password' =&gt; 'password',
  'prefix' =&gt; array(
    'default'         =&gt; 'site1_prefix_',
  ... etc


and the site name:

$config['system.site']['name'] = 'Site 1 Name will override default site name';


of course YMMV

tl;dr - yes you can, but be aware of the order in which your PHP will execute and apply the settings