"Could" is a modal verb in English that has various meanings in various sentences. This is more of a grammar for usages of modal verbs that you could have found by searching, but anyhow, I always find subscription-only LDOCE pretty helpful, so I'm quoting its definitions here:
Past ability: used as the past tense of 'can' to say what someone was able to do or was allowed to do in the past:
teacher said we could all go home.
I couldn't get tickets after
all, they were sold out.
a. used to say that something is possible or might happen:
Most accidents in the home could be prevented.
b. used to say that something was a possibility in the past, but
did not actually happen:
I could have warned you if I had known
where you were.
Emphasizing your feelings (spoken): used to emphasize how happy, angry, etc. you are by saying how you want to express your
I was so angry that I could have killed her.
Requesting (spoken): used to make a polite request:
Could I have a glass of water, please?
Suggesting: used to suggest doing something:
You could ask your doctor for a check-up.
Annoyance (spoken): used to show that you are annoyed about someone's behavior:
You could have told me you were going to be
late (=you should have told me, but you did not)!
The last three aren't all that much related to your sentences, so I'm not putting them here, and you can always search and find out more about them as your homework!
In the quotation you've provided, the first three imply the possibility (2.a.) and the three others can fit in 2.b., 3, or 6, however if you search and read some grammar books, you'll find out more about could have or if clauses (4th could in your quotation).