The source offered for "most famous" is the Scottish National Gallery's website description. But that says nothing about the painting being the most famous in the series, neither does it make a remark about the painting giving an impression of the constant motion of the water nor for that matter mentioning the rainbow you can see in the bottom right corner. These appear to be Hafspajen's observations. Assuming that Hafspajen made an internet search, as she surely did, on the terms "Niagara Falls from the American Side", she would have been directed first to American at the Scottish National Gallery (SNG) and then secondly to Niagara at the National Gallery, Washington (NGA). This latter source, which she must have looked at, devotes its final paragraph to a description of the phenomenal success of the 1857 Niagara painting. Yet Hafspajen persisted with her description of the 1867 American painting as the most famous of the series. Source  is available as a Google book and is presumably the source for the two month observation about 1857 Niagara. This would also be the source for the title Niagara Falls, as the book gives that as the original title. But the same source indicates the phenomenal success of the 1857 Niagara and goes on to say it was exhibited at the 1867 Exposition and that by that time it was known simply as Niagara. It calls the painting "famous". It then goes on to describe the 1867 American, making some interesting comments about its composition and provenance that Hafspajen ignored, preferring her own trite unsourced observations about the water appearing to be in constant motion and the rainbow.
Hafspajen eventually added a blog as an External Link. This blog adds nothing to the discussion and shouldn't have been offered as an external link. It incorrectly calls the 1867 painting Niagara Falls and we suspect this was Hafspajen's original source.
The remarks about aesthetics we shall leave to the DYKs, where the question of whether they were copyright violations was raised. But we can give an example of the problem right now with the sentence "Emotions like awe— especially that which is experienced in confronting the sublimity of untamed nature and its picturesque qualities were new aesthetic categories, and very different from Realism and Classicism as a source of aesthetic experience." Conoisseurs of Hafspajen's unique literary style will recognise this lacks her distinctive touch, save perhaps in its minimalist punctuation. On the other hand there seems to be nothing in the sources offered that expresses those thoughts. In fact it's copy-paste from Wikipedia's own article on Romanticism.
We should look at Sagaciousphil's and Weller's contributions to the article. The in creation template we mentioned above stipulates that is should be removed if the article hasn't been edited for a few hours. For this article, that would have been after Weller's 17:57 8 October 2014 edit and we can take this as the cut-off point to judge how effective the collaboration was. This was the state of the article at that point, after some further 40 or so edits following Hafspajen's.
Nothing much has really changed. Sagaciousphil made what she described as a "very light" copy-edit. She left untouched the choice sentence commencing "Emotions like awe ...", possibly because she had difficulty, as we do, in construing it. Weller corrected the error of date we mention and introduced another source for Edmund Burke. Their contributions for the most part, however, were essentially housekeeping, and indeed the article at 26 November 2015, when Marinka edited, was essentially the same article as Hafspajen left it 7 October 2014. At one point Hafspajen introduced a whole raft of strange stuff in the notes, eventually removed by a fresh editor (Mandarax) "with permission" (it would have had to be if Mandarax were to continue entertaining ambitions of a peaceful end to life's great adventure, at least in Wikipedia).
The most notable feature is the strange reluctance properly to indicate the importance of 1857 Niagara, still one of the star attractions at the National Gallery of Art, Washington. Not even a link is provided. It would have been a matter of moments to correct. Here is Marinka's eventual edit: