I think it would have been almost impossible to challenge Russell's ideas because who could keep up with everything he was writing? I imagine that any loyal readers were just hanging on for dear life trying to keep up with all the claims he was verbosely making in his Studies volumes, the magazines, etc. I haven't read a lot of his writing, but it seems like Russell's ideas were constantly mutating. So before you could challenge his writing in any large-scale way, you would first need to assemble it into a coherent worldview (as of a certain point in time), and at that point you'd still need to find a way to test it against reality.
And after all, who could really say if Portugal was one of the horns or not? There was no basis for saying it was incorrect except to say that your own common sense found it absurd, or that you didn't personally believe that Revelation had anything to do with the 16th-20th centuries. The only way you could disprove him is if he made a prediction, or his work made an implicit prediction, which did not come true. The last that Russell knew, his 1914 prediction may have at least partly failed, but he still expected to see the end of the world before the Great War was over -- and then he died in 1916.
The same issue is probably the case with Rutherford's "colorful book" period. There were so many claims being made, with multiple books published per year, that I wonder if anyone ever bothered to assemble them together and try to debunk them (not that I think it's worth the time). All you can do is kind of back away from the details and try to look for something to test, or question the overall dispensationalist approach to scripture, but it's not really possible to challenge a myriad of little subjective interpretations. Likewise, it seems that he may have expected the end of the world to come before World War II ended, but he died first in 1942.
I think it's easier now to debunk the teachings because, despite some claims on the forum that the religion is changing rapidly, the teachings have been fairly locked down for a while now and most of the literature is devoted to repeating the same doctrine over and over. Also, we have the advantage of over a century having passed since Russell started writing, which allows us to see a collection of wrong predictions that the Society has made. If I had lived back in Russell's time, I don't know if I would have been frightened by his craziness or impressed by his Bible interpretations, but things are much clearer now.