In my 40+ years as a master mechanic I have replaced many oil pans and rear seals and have followed all of your guidelines and even tried a new "China" oil pan. I have finally sourced the leak to the rear main, new seal didn't fix it. The old seal just slid right out and the new one just slipped right in. Generally they have some friction and you have a tough time getting them in. I was just hoping for a silver bullet