I have followed the discussions over sideslipping with great interest. I have been flying for almost 50 years, mainly in the UK, with much of this time being involved in flight test and instruction and I had never before heard the phrase 'forward slip'! So, a few thoughts from my background on these discussions.
'Sideslip' is defined as the angle between the relative airflow and the aircraft's longitudinal axis. The manoeuvres flown on an approach involving the use of sideslip to generate drag are 'steady heading sideslips', a phrase that is used for a specific flight test technique but also is valid in this context. A rudder input generates a sideslip angle and a yaw rate ,and the yaw rate is reduced to zero (ie. a constant or steady heading) by the use of bank angle in the opposite sense to the applied rudder. Note that all of the discussion here is relative to the aircraft as the frame of reference and its motion with respect to the airmass. The flightpath vector of the aircraft can then be resolved with the wind vector to generate a velocity with respect to earth axes ie. where it is tracking over the ground. The pilot can control the path over the ground by varying the rudder input or bank angle in order to turn the aircraft and when the desired track is achieved he can re-establish the sideslip and bank angles to maintain the desired path with zero yaw rate/on a constant heading. Aerodynamically, this is a single case. What has been discussed in many of the earlier posts is just the difference between the frames of reference ie. the air mass or the ground. In my opinion, the use of the phrase 'forward slip' only serves to cause confusion.