sounds like crap to me, I've done plenty of accellerating in 4th & 5th and theres never been a problem on mine and VSS feed has never been connected. as you say, how will it know its in 4th & 5th anyway? My thoughts were it was to limit the top speed, but only a guess.
28 - lambda heater relay trigger, only present on early models. later models the lambda just gets power direct from fuel pump relay. if you have a relay and no pin 28, the last pin on relay will be an earth
20 - lambda sensor live (black wire on lambda)
42 - lambda sensor earth (grey wire on lambda)
65 - shield for lambda sensor wires
all abfs use a 4 wire lambda sensor

large brown/blue sensor -> low oil pressure switch, blue/black wire
large white sensor -> high oil pressure switch, yellow wire
small white sensor -> mfa oil temp, green/black wire