JWs will exploit any and every frivolous technicality to defend the org. On a Y!A question about JW false predictions, I saw a JW respond by saying that JWs never made any false predictions - that such predictions were make by Bible Student groups before the group named Jehovah's Witnesses came into existence in 1931. Yes, they will stoop that low.
When mentioning the 1975 issue, it's best avoid saying or implying that the org explicitly predicted the end in 1975. Instead, it would be better to say that the org. "implied" or "insinuated" through innuendo that the end would come in 1975. Such a claim would be more difficult for a JW to weasel out of.
Keep in mind too, what the org says about Satan's words to Eve at Genesis 3. This could prove very helpful for exposing the hypocrisy of the "the org never explicitly said so" defence argument employed by JWs. How so? Well Satan never explicitly told Eve that God was a selfish ruler withholding good from humans and that humans are better off ruling themselves, did he? Yet the org. reads those insinuations into Satan's words and charges him with being just as guilty as if he had explicitly said it! So since the org. judges others for what they do not explicitly say but insinuate; then it is only fair that others also judge the org. for what it does not explicitly say but clearly insinuates. They clearly insinuated that the end would come in 1975!