Its been noted on this website and the AHK website that AHK could be used to the extent that it is illegal. Just don't use it that way.

People can change the argument so that AHK or even hardware based setups to multi-box are against the TOS of WoW. WoW TOS is so global that it's silly, they can ban you for no reason at all.

How many people do you know that use a G15, Nostromo or other type of hardware? According to the ToS they should all be banned. Blizz isn't going to do that. I'm willing to bet more then 30% of the player base uses these items. These are all considered third party software.

Using AHK to send 1 to three different windows still involves you pushing that 1 key. It requires you to be at the computer and in control.

To answer your AHK question though, you can specify that certain keystrokes only goto a certain wow instance.

[code:1]0::
KeyWait, 0, D
ControlSend,, 0 , ahk_id %wowid2%

Return[/code:1]

The code above would only send the keystroke to your second wow instance.

According to your argument though, you can't even do this. You are using a third party program to aid you.