How can I manipulate several security settings from terminal?
1) Auto-login

That one is tricky. The default is saved in

defaults read /Library/Preferences/com.apple.loginwindow autoLoginUser

But in order to turn it on or off, you need to do it as root.

Set it:

sudo defaults write /Library/Preferences/com.apple.loginwindow autoLoginUser ShortName

Delete it (turn it off):

sudo defaults delete /Library/Preferences/com.apple.loginwindow autoLoginUser

2) Showing password after screensaver and sleep mode

I've been trying to get that one to work, and I can't

You will see a lot of hints telling you that the answer is

defaults write com.apple.screensaver askForPassword 1


or variations like

defaults -currentHost write com.apple.screensaver askForPassword -int 1


and that should work, because if you turn it off via System Preferences, you will see:

% defaults read com.apple.screensaver
{
    askForPassword = 0;
    askForPasswordDelay = 0;
    tokenRemovalAction = 0;
}


and then if you turn it back on via System Preferences, you will see

% defaults read com.apple.screensaver
{
    askForPassword = 1;
    askForPasswordDelay = 0;
    tokenRemovalAction = 0;
}


BUT if turn it OFF and the quit System Preferences and change the setting using 'defaults write', when I re-launch System Preferences, it does not reflect that change.

I'd really like to know the answer to that one (preferably without osascript, but if there is no other way, I'll accept it).

3) Go to sleep mode after x minutes

Assuming you mean "have the computer go to sleep after x minutes" you want:

sudo pmset sleep 20


You can also use different settings specifically for when you are on battery (for MacBooks):

sudo pmset -b sleep 10


If you want to specify never sleeping when plugged in, use

sudo pmset -c sleep 0


4) Enable screensaver after x minutes

@Daniel's recommendation worked for me:

sudo osascript -e 'tell application "System Events" to set delay interval of screen saver preferences to 30'


You can use 'sudo pmset displaysleep X' to have the display sleep instead of using the screensaver.