Can multiple authors be assigned on a single custom post?

Easy way: use this plugin https://wordpress.org/plugins/user-role-editor/

More complicated way: You can use functions.php to add all of the custom capabilities to a specific roles. But ofc you need a name of these capabilities.

This code is usefull if you want to add X same capabilities to X roles.

Add to functions.php

   function wphm_add_custom_capabilities_to_roles($roles, $capabilities) {
     foreach($roles as $the_role) {
       $role = get_role($the_role);
       foreach($capabilities as $capability) {
         $role -> add_cap($capability);
       }
     }
   }

  wphm_add_custom_capabilities_to_roles(
    array(  // these roles will recieve capabilities
      'custom_user_role_wphm_support',
      'administrator'
    ),
    array( // capabilities
      "read_wphm_custom",
      "read_private_wphm_custom",
      "edit_wphm_custom",
      "edit_others_wphm_custom",
      "edit_published_wphm_custom",
      "publish_wphm_custom",
      "delete_others_wphm_custom",
      "delete_private_wphm_custom",
      "delete_published_wphm_custom"
    )
  );

To remove a capability with this loop just modify a line $role -> add_cap($capability); to $role -> remove_cap($capability)