I found several people who departed from the suggested colourway and their versions looked lovely. The original was stitched in 1790 when there was not the vast array of colour choices available in silks that we enjoy now. So while there is varietion in colour, some of it looks muddy. Then again, maybe the original colours were a lot brighter and better defined, it's just that age has robbed them of their glory?
I also found a few people who corrected the mistakes in the original pattern, producing a more symmetrical result. Others kept the mistakes in, viewing them as charming rather than irritating. I guess it depends on whether you believe you are stitching your own version, or a reproduction of an original.
I'm putting off starting my Mary Wigham until I decide in which camp I am. I know that I am the sort of person who see's the mistakes, especially in patterns of such symmetry. Slightly skewiff pictures irritate the life out of me, and I checked all our patterned mugs before purchase, for off-centre patterns and other design faults/blemishes. So much so, that the ladies of the china department in Lewis' think I am nuts!
I have made mistakes in my own stitching (who hasn't) which has me unpicking and grumbling, even when the mistake is so small that it could easily be incorporated into the pattern without too much bother. So if I do start stitching this is will be a huge wrench to be deliberately doings things wrong. But maybe that's a lesson I need to learn ...