On the Permissions screen at /admin/people/permissions, you can give the permission Administer users to the Manager role.
Administer users is an administrator permission, but you do not need to give the other administrator permissions to the Manager role.
If you really don't want the Manager role to have unlimited access to user accounts, you could use the ...
Nope, new users' one-time-link do not expire. Only "forgot passwords"-links do.
Also, see this discussion. It might come in future Drupal 9 releases. https://www.drupal.org/project/drupal/issues/3097238
Each Comment Type can only be applied to one type of entity (nodes (content), users, and taxonomy terms are examples of different types of entities).
By default, Drupal core only provides a comment type for nodes.
So, to add comments on users, you will need to create a user comment type.
Go to /admin/structure/comment.
Select Add comment type.
Add a Label ...
Yes, you can use the user_cancel process to prevent content being deleted. As you found out it doesn't help to remove the options from UI. Programmatically you need to call the function directly, specifying the method user_cancel_reassign to replace the author with anonymous before deleting the user:
user_cancel(, $uid, 'user_cancel_reassign');
$account = ...
With twig_tweak (for twig token filter, tested on 8.x-2.9) you can add a token based link to the user account menu in Drupal 8/9 (/admin/structure/menu/manage/account).
E.g. A menu link:
Also works with multiple tokens.
title: [current-user:name] and [current-user:created]