Vanderhoven, Rutherford's flash of inspiration as you put it, has an interesting origin which I found in a book in the British Museum Library. (Religious Sects by B WIlson 1970 Wiedenfeld and Nicholson p.194)
I suggest there is no tangible evidence for such
thing as a ”great crowd” or “other sheep” they are simply evolved religious fictions
but the development of JW doctrine is interesting nevertheless ...truth on the other
hand never needs to evolve.
Since dogma doesn’t grow on trees; it is nearly always
borrowed from earlier versions, often recruited to serve a new purpose but in this case Rutherford stole the idea to meet the requirement of a failing older teaching about the 144k which was becoming implausible, for it was projected that soon there would be more than that number of JWs! How then to fudge the figures?
An earlier
incarnation of two hopes was proposed by a Watchtower Bible Student called Alexander Freytag who after the failure of the 1914 teaching and the death of Russell, looked again at a new meaning in the scriptures. Being a threat to Rutherford's imperial power ambitions, Freytag was expelled from the organisation. As B Wilson put it:
"Freytag intending men to overcome death and separating after the 1914 disappointment, started a new sect based on altruism borrowing from Spencer and Kropotkin...i.e. health and millennialism, the law of just deserts".This included the groupings of a heaven-bound little flock (petite troupeau) and a great crowd (Armee de l'Eternel) who would live on earth.
This splinter group, which I believe still exists today in continental Europe, mangles Russellite teachings with a health-conscious, non-smoking life style. As with Russell and the Watchtower control, Freytag's death was marked by an acrimonious leadership takeover and yet further splintering.
Freytag's concept of 'two hopes' was hastily re-cycled to resolve a doctrinal failure in 1935 by the crapulent Rutherford.(I think crapulent describes him well!) Freytag’s doctrine was foisted on the newly named
Jehovah's Witnesses... and the dumb sheep followed the drunken shepherd.
So there it is, Rutherford stole the idea of two hopes from an apostate.