I agree with all that you say Arne, except your conclusion.
My first thought was that the “Marshall Johnson” painting was made for a children’s story book. It is indeed disproportionate, the freeboard and sheer line are a little exaggerated. The bulwarks and rails look laughably “agricultural”. The Donnelly drawing is much more believable.
The “Marshall Johnson” painting has some correct details – too correct for an artist of 1880, which was what first led to my suspicion that it may have been painted much later than 1880. But there are other details, particularly in the rigging, which are clear, but show that the artist does not understand – for example the main lift/lazy jack which looks like a running backstay – another example is the odd structure on the deck amidships, on the painting, which is probably a completely misunderstood winch gear. These same detailed items appear also on the Donnelly pen-and-ink, if you enlarge it, but executed differently as Donnelly knew what they were.
The incorrect proportions are to be expected from an artist who does not understand what he is painting. (Note also, the painting was not photographed square-on which may have foreshortened it in the vertical direction, although I do not think this factor counts for much.) When marine artists in the UK were first confronted by a Chinese junk (never before seen in the UK) this was what they made of it. This was how four different marine artists actually saw the Keying in the 1840s.
My mother was something of an artist and could make quite good landscapes, but she had no understanding of boat design and could never get a boat to look right – she couldn’t see a boat in the way you or I would see it. My guess is that the creator of the “Marshall Johnson” painting tried to copy the Donnelly and saw certain details but did not understand them. We tend to forget that most people have never looked at a set of lines, have never considered hull structure or the function of sails or rigging, and would have difficulty making a photographic image of something as complex as a sailboat which would be satisfactory to you and me.
Yet if you go back to the original files, as posted by Shemaya and David, and blow them up for close study - there are some remarkably similar features, too many for coincidence (for instance the furled lower main panel, the number of panels, the textural patterns on the topsides to name a few), which leave me convinced that the two pieces of art do represent the same boat, and that the “Marshall Johnson” must most likely be an attempt at copying the Donnelly. I’m happy to be proved wrong and I do hope more information turns up (perhaps from Carolie).