Most Stainless fittings and a lot of PVC comes from China and the threads are questionable. If the Oatley Great White didn't work I would try the "soft-set". Really sorry you are having this problem as it should be a non-issue.
Thanks for the kind words about the girls who run the place. One is my wife. One has been with me so long she treats me like she is my wife. And the other is my Daughter In Law. My son and I just do what we are told. Funny many people call just to hear their Texas accent. We used to try and hide our accent, but now we think of it as a badge of honor. We may talk funny. But we can normally fix a water system. Yours seems to be the exception and we all fell bad for your trouble with the thread leaks.