If the airplane is in the attitude for "zero sideslip" I do believe the string would go straight back...
I'd go along with that ..
am I missing something?
yes -
His view is that during initial ME training, the ball was kept half out and a bank of 5 degrees towards the good engine was needed.
noted and quite appropriate for speeds around Vmca
This would keep a yaw string (if installed) right down the centerline of the aircraft.
unfortunately, no - the previous description does not define nil slip. With 5 degrees bank, you would expect there to be some slip into the live engine and the yaw string to be off centre to the dead engine.
every time I hear an instructor teach that you should slam rudder in to make sure the nose doesn't move a hair off your heading when you stall
putting the colourful use of "slam" to one side, the technique should be to use rudder to the extent necessary to maintain a constant heading. All part of the desirable aim to minimise the likelihood of a departure.