I remain to be convinced about the wisdom of splitting the 42A at East Didsbury, although I would agree that an increase to every 20 minutes, along with a similar increase on the 42 from Stockport, would be a good move. This might allow for a slight reduction in the 142 frequency, while removing the 42A as you suggest would quite likely require an increase in the 142. Incidentally, would you also wish to split the 42 at East Didsbury - the same argument applies, surely?
I was in the Stockport schedules team that introduced what is now the 42A pre-deregulation. It was then a plain 42, an extension of one of the two buses per hour that ran Piccadilly - East Didsbury, the third one per hour was a 46 that ran to Millgate Lane. The extended 42 replaced the 309, Reddish Houldsworth Square to West Didsbury, which was a pain to operate, requiring dead running to and from Reddish to relieve drivers after every couple of hours work (admittedly the extended 42 also required this, just less often as Stockport had just one bus on the route, Birchfields Road garage covering the others). The 309 ran every 30 minutes, the 42 just hourly (these were tough times) but an additional 309 service was initially retained at peak times and Saturday shopping hours to placate the Trade Union. I can't find a Bus Guide for this in my 1985 set, so I suspect it was soon withdrawn.
As well as operational efficiency, the justification was providing at least a token direct service to the Manchester hospitals campus from the section of route through The Heatons. The link to Didsbury village was already long-established via the 309 - this would be lost if the 42A was split at East Didsbury, and I think this would be an extremely unpopular move. Forcing passengers to change a few stops short of their intended destination seems hard to justify in a suburban location where the link has existed for many, many decades.