How would I know if the insurance is going to cover the endorsement that I did?

I was the driver at time of the accident. I have a 23 year old passenger with me. He doesn’t even have a license. The insurance now sent a letter that I have an unlisted driver and it’s said add or exclude driver. I haven’t listed him because he just moved in my house and he doesn’t drive. Most of the time, he rides the bus. I went to my broker, made an endorsement and added excluded driver. I don’t know why I have to put him as a driver as he was just the passenger. I mention my nephew because they asked where I was trying to go. I told them to let to my nephew at his job and now they send me those letters. I understand that to exclude my nephew means if he drives my car in the future and if he has an accident, the insurance won’t cover that. I did that endorsement from the accident I had. I was driving. The agent set him as excluded driver but I worry if the insurance is going to cover that accident or not. My car was in the auto body shop for 15 days.