Could do vs Could have done (past possibility)

1) Do I understand it right?

I could start a business= Nothing stopped me from opening a business: maybe I had enough money and good ideas. It doesn't matter whether I started it or not.

Possible sentences:
I could start a business in 1999. But I didn't and I regret that I didn't.
I could start a business in 1999. And I did. Now I am a famous businessman.

I could have started a business = I could start a business 10 years ago, but I didn't.

Possible sentences:

I could have started a business in 1999, but it was too dangerous.
I could have started a business in 1999 and I regret that I didn't.
I could have started a business in 1999, and I did. (incorrect?)

2) I came across an interesting sentence from the book ""Stone of Tears" by Goodkind Terry. Here it is:
There was only one thing you could have done to prevent losing it. And you have done it

There is another similar sentence:
You did the best you could have done

They have made me confused. Grammar books state that we use "could have" to talk about things that didn't happen. Why was "could have" used in these sentences then? Should it be "could" instead of "could have"?

Thank you for your help.