It is many years since I have dealt with Rob Elder and even then that was in his previous life. Unless he has changed his spots in the meantime he was, and I presume still is, a reasonably decent chap with whom to interact.
I think that the point which needs to be kept in mind is that he is an administrator and government policy specialist (and, indeed, a man of considerable knowledge and expertise in those areas), but not an aviation engineering or operational technical specialist .... if you want a technical answer, then ask the appropriate technical specialist ... if you ask Rob he is going to do just that in any case I suspect ....
The introduction of local flight manuals was in response to a perceived need for additional data over and above what was in the CofA and placarded data. What was quite workable for a Tiger was not very suitable for the crop of larger and more sophisticated GA machines which came on the market during and since the 60s. There was nothing at all sinister or conspiratorial about the introduction of light aircraft flight manuals.
In recent times the regulatory philosophy has changed to one of automatic acceptance of foreign NAA approvals whether we like it or not and, given that the foreign flight manuals are now of a higher value than in the bad old days, it was inevitable that the local animal, with its cost overheads, had to go. Again, nothing at all surprising about that.
There is nothing to stop a pilot or operator using the foreign unfactored POH or AFM data as a basis for generating conservative data similar to the old P-chart data ... the factors used are well known and the standard procedure used to generate the charts was published in a DCA airworthiness document which was generally available to the engineering fraternity ... if you want the equations used, I am sure that I, or others, could be persuaded to dig out that data from the dustier parts of our filing cabinets for you ... mind you ... a Guinness or three would improve our enthusiasm for battling the spiders in the bottom drawer ....