GRUB를 편집하여 분실한 계정 비밀번호를 변경했지만 여전히 계정에 액세스할 수 없습니까? (리눅스 민트 18.3)

Linux Mint 18.3을 새로 설치했습니다(시스템 설정: Windows 10을 사용한 이중 부팅).

저는 4개의 계정을 설정했습니다:

user_1
user_2
user_3
user_4

비밀번호를 적어두지 않았고 다음날 아침에 일어났더니 하나도 기억이 나지 않았습니다.

나는 다음과 같이 네 명의 사용자 모두의 비밀번호를 변경해 보았습니다.

(1) Booted my system and when the Grub menu appeared, 
    I made sure my Linux Mint 18.3 OS was highlighted,
    I pressed e (for edit).
(2) Arrowed down to the line that starts with "linux"
    It looks like this: linux /boot/vmlinuz-3.16.0-38-generic, 
    root=UUID=b1bde976-50e2-4c32-a760-17b091b4202f ro quiet splash 
    $vt_handoff
(3) At the end of that line I deleted the words,
     "quiet splash $vt_handoff" if that is there, and put: rw init=/bin/bash
(4) I then pressed F10 to boot. 
     This resulted as a boot into a root shell.
     I did an ls /home and the system outputted the following:
     user_2
     user_3
(5) I entered a new password for both these users as follows:
    passwd user_1 (and when prompted twice for the password, 
    I entered the new password both times)
(6) I then entered the following command: sync
(7) Next I entered the following command: reboot -f

Note: I also did the same as step (4) step (5), step (6) and step (7) for root

지금까지 user_1, user_2 및 root의 비밀번호를 성공적으로 변경했습니다.

그러나 아직 로그인 화면에서 user_1 및 user_4에 대한 올바른 비밀번호를 모르기 때문에 로그인할 수 없습니다.

내 질문:

(a) How do I change the passwords for user_1 and user_4?
    I assume both are administrative users, especially since it is,
    my belief that the first user created after a fresh install is,
    an administrative account by default.

(b) Why when I log in as user_2 or user_3 and do an ls /home does it only show me, 
    the user accounts for user_2 and user_3.

(c) In general, should I only have one admin account on my system?

(d) Does an administrator account have the same privileges as root after,
    (su "ing" subsequent to logging, into user_2 and user_3)