this isnt copy/pasted so it might be a bit off, but what youll really want is
/cast [target=help] Intervene; [target=targettarget,exists] Intervene
works on friendlies, works on enemies, and has some protection against getting the click-target cursor if you dont have a target or your target doesnt have a target.
unfortunately there is no macro that will make you even a mediocre tank. i would suggest researching if its possible to do something like this:
/logout
/deletecharacter [target=player]
/castrandom rollhunter; rollrogue; rollmage; rollwarlock
that way mashing one button will still make you suck at wow, but at least you wont be such a huge liability to your group.
(no offense to the pure dps classes, its just easier to cover for a crappy dpser than it is to cover for a crappy healer or tank
)