If they want to hark on about Rutherford's reasons for branding the fallen away Bible Students as "evil slaves" then that leaves prime opportunity for you to analyse Rutherford's reasons and ask how could it be that tens of thousands of Bible Students, who the Society says were all anointed in 1919, suddenly turn from being the only approved Christians on planet earth into "evil slaves" in 15 short years or so. A reasonable minded JW (which most of my relatives and friends have been, thank goodness) can see this can seem rather unreasonable. Point out that if Russell had lived for a few more years and Rutherford had taken control, then Russell would undoubtedly have been one who would have left along with all the other tens of thousands of Bible Students? If the JW doesn't agree with this hypothetical proposition, then pull out some quotes of Russells was opposed to highly centralised control and the entire concept of "organisation", so he would have been opposed to Rutherford's new strict control and increasing ruthlessness. If you produce the exact quotes of Russell on this to make it clear that Russell would have been disturbed at the new direction Rutherford was taking.
The Watchtower teaches that the "evil slave" was allready at work before 1918, but that in that year they finally where designated as such. Anybody that would have left the Watchtower as left by Rutherford (and yes there where many) would still have been put in the catagory of evil slave.
In the context of Watchtower thinking the membership is never as important as the leadership. I think this is what you are forgetting. Just like Ray Franz points out in one of his books (can't remember which one) whenever the JW membership hears "Faithful and Discreet Slave" they do not think of all the anointed people on the earth (what's left of the 144,000). Instead they think of the magazines and books that come from the authority in Brooklyn. While thinking of it in terms of the remnant 144,000 is more accurate when it comes to their theolgoy, it is what it means in terms of authority that the adverage JW really understands.
They simply don't view all those people as being collectivly selected. The Watchtower has had it's cake and eaten it too with this concept for quite a while. I certainly don't think showing it to a JW is part of 5 easy steps that can "destroy their faith"