Hi Gail,
This is sounding tough for you. I really get that. However, by asking the GP and the Optometrist to take the action you are putting them in a very difficult position. Tough, but true.
When we were going through this journey with my mum I asked the GP to help. He, as yours did, ask my mother to do the driving assessment. She point blank refused. He also asked her to consider giving up driving. Again, point blank refusal.
I did take things in my hands as sibling was pfaffing. I decided I could not live with myself were mother to hit someone. She had clocked up quite a bit of superficial damage on her car. So I wrote an approximate diary of what the damage was and when it had occurred. Stating mother had refused to take the driving assessment. Sibling agreed to the action.
The DVLA wrote to my mother rescinding her driving licence. There could be no argument on the basis of the letter. It was the DVLA. I was not mentioned. However, I suspect sibling may have dobbed me in it. My mother asked recently whether I had informed DVLA. This is a woman with a 5 second memory. Her licence was taken away over 1 year ago.
I do no have quite the family political situation you have. It must be heard being stuck between a rock and a hard place. Your dad has been advised not to drive. He will not be covered by insurance should he drive. It will take a little, not much, time to recover from cataracts. So either a case of writing to the DVLA now or after the operation should you still feel he is not safe.
By giving the responsibility to the GP or the optometrist, as a friend pointed out to me, it is asking them to play God. And that is not fair.
Best of luck in your decision.