Hi oompa
You mentioned that when you try to bring up some of the Society’s pre-1919 teachings that have changed, your dad brings up some of their teachings that have not changed and then he says, "The Society was more correct than anyone else."
You then asked, "How to get around this?"
Page 40 of "Captives of a Concept" says this: "There are some teachings that have not changed since 1919. They include: God’s Name is not "God," God is not a Trinity, the soul is not immortal, hell is not hot, the dead are not conscious and the earth will not be destroyed. But there were others who taught these same things.* If these teachings would have passed an examination by Jesus they would have passed no matter who was teaching them."
*A footnote explains: On page 622 of the "Proclaimers" book it acknowledges that Russell was not the only one who taught these things but it was just that he did a better job of publishing them than anyone else. See also page 45 under the heading, "Influence of Others."
What the "Proclaimers" book acknowledges (above) may help you "get around" your dad's present argument, but unless he is ready to face the possibility that the Society may not be "Jehovah’s organization" he will likely come up with some other reason (like Proverbs 4:18’s "new light") that will allows him to continue to believe what he wants (or needs) to believe no matter the Society was or wasn’t teaching down till 1919.
To help keep me from getting frustrated when this happens I always try to keep in mind what Paul explained to Timothy when he said how we need to be as gentle and mild as we can when presenting what we have learned in hope that "God may give them repentance leading to an accurate knowledge of the truth" about the Watchtower Society "and (then) they may come back to their proper senses." – 2 Timothy 2:23-26
I was surprised to notice that it isn’t the evidence that we have that will lead them to the knowledge of the truth. It is God giving them repentance that will do it.
Don Cameron