Hi Ihsan,
talking on your first kind of help, you will need something which will not just only multiplex between keypad and LCD but also make sure that both of them remain isolated from each other. Even a small interference from keypad can affect LCD’s working. If you are thinking of using latches then according to me at least two of them will be required, one for the LCD and the other for keypad.
And as far as your second help is concerned, these following two links will be enough:
“engineersgarage.com/microcontroller/8051projects/electronic-code-lock-with-password-using-at89c51-circuit”
and
“engineersgarage.com/contribution/keypad-locking-system-with-user-defined-password”