Merlin drawings
There was never any need to 'redraw' the Merlin drawings. A set of linen copies known as 'brown lines' were probably made from the original R-R tracings and sent to Packard, and these could be easily altered to add to or change any feature of the design. In 1952 when I was working in the drawing office of the North British Locomotive Co during my university vacation I was put on a job which the NBL got when the Indian customer ran out of dollars and had to buy British. This involved taking a set of brown lines sent to us by the Baldwin Loco Works in the USA (the original manufacturer) and altering them to permit British fasteners to be used - the exact opposite of the problem Packard had. We carefully erased all the American data and replaced it with our thread standards . There were other changes, mainly to materials, needed as well. It was a really boring job.
As a mechanical designer all my life, I have to doubt very much the story about the tolerances. Stanley Hooker's book is not the most reliable source. Try Peter Pugh's trilogy 'The Magic of a Name" for a more balanced view of what went on in R-R. In my time (1956-1959) in the Drawing Office at Elton Road, engineering tolerances were dealt with by a dedicated team of specialists.