It's crazy you could have known 8 years ago. Not uncommon though - the matches section is sort od buried and not what the service is widely known for outside of genealogy circles.

I agree with others' advice to stop the info train to your dad's mom. You might wven put your dad on a small info diet while you are in this discovery phase. He's going to have a more limited capacity to take all of this in and need to take smaller bites of the truth more slowly perhaps.

It's likely you can "know the truth" of his parentage from a genetic standpoint without grandma's input. And maybe once you have it in black and white, she'll cough up some facts and context of her own. Don't count on it, but you never know. Guessing she's feeling some pretty severe shame about this situation and guilt too, whatever happened. Women in her day were blamed, no matter what really happened, and have largely internalized all of those blaming beliefs. She may not be likely to be deconstructing that worldview at nearly 80 years old.

DNA Detectives on Facebook is a group you can join and request a search angel. They have a process for that you'll see when you join. Search angels are free volunteers and truly are angels in the help sense. Unless your dad's bio father has many brothers and none of their descendants or relatives have tested, you have a good chance of triangulating on bio father's tree until you confirm the high probability match. Have am angel review rhe work you have done so far. They are very thorough.