You are right in preferring "busy-ness" as the collective noun for ferrets. It is the modern form of the word "besynes" which appeared in the "Book of St Albans" of 1486 as the "proper term" for ferrets, i.e. a word which describes the characteristics of ferrets.
Over the centuries misunderstandings about what these proper terms were led to their all being regarded as collective nouns even where, as with a "singular" of boars it was patently obvious that the term could not possibly be a collective noun.
In the case of ferrets the word "fesynes" arose through a transcription error in 1801 when the list in Joseph Strutt's "Sports and Pastimes" failed to copy correctly earlierlists.
The matter was investigated thoroughly by the scholar John Hodgkin in the early years of the twentieth century and published in the paper 'Proper Terms: An attempt at a rational explanation of the meanings of the Collection of Phrases in "The Book of St Albans," 1486, entitled "The Compaynys of beestys and fowlys" and similar lists.', Transactions of the Philological Society 1907-1910 Part III, pp 1 - 187, Kegan, Paul, Trench & Trübner & Co, Ltd, London, 1909.
