Earlier quoted context omitted.
Yes, obviously you can as admin but that's just the admin power. You can also delete/edit your own messages in groups but they will be visible in the admin log for 2 days. So these 2 features are not meant to remove your messages from being stored/archived. Its just the normal moderation feature of group chats.
It's trivial to give everyone in a group admin rights with only the permission to delete messages. You can also delete all messages in a group from a specific user in one action.
no, its not. you can not do that there is an admin limit 20 iirc