Not gonna lie... having reference to the Dimond Brothers does NOT help your case any.
Aside from Creation and a few other basics ("a broken clock is right twice a day", plus we Catholics agree with Mormons, Baptists, and many other heretics on countless elements of the Natural Law), they get just about everything else wrong. They are the modern-day Pharisees, with all the same faults Our Lord condemned the Pharisees for: Making up laws, condemning others unjustly, rash judgment, cutting themselves off from God, forgetting Charity, etc.
If you want to make your point, I would avoid bringing the Dimond Brothers into it. Unless you want to have the opposite effect :)
I'm not kidding. The Dimond Brothers are SO bad, that if a Catholic immediately dismissed your criticism of NFP as "Oh, that's more Dimond Brothers extremist nonsense" THEY WOULD BE JUSTIFIED BEFORE GOD IN DOING SO. Even if that was their only reason.
If I dismissed something being pushed by (((Red Sea Pedestrians))) and God wouldn't hold it against me for failing to look into it, the same would apply to other villains like the Dimond Brothers.
After all, when a mouthpiece has been proven vile and wrong 100 times, you can be at least MORALLY certain (maybe not metaphysically or scientifically certain) that they are wrong the 101st time. It's about moral certainty -- enough to base one's actions on.
I think they have excellent resources on a wide range of topics, from evolution to "Orthodoxy". And they are the most eloquent of the few that denounce NFP and BoD.
I understand people don't like them, and it is justified to an extent, however, I definitely don't agree they are nearly as bad as you seem to think.
I'm sure people hate them in part because of their strict views on NFP, so it's a chicken and egg problem.
I'd like to see what is the vilest thing the Dimonds ever did. Do you, or someone else, have a link?