"Could" is about a hypothetical, or an option. It exists in the present.
"Could have" or "could've" is a hypothetical or option that only exists in the past.
- She needed money so much, she could even have worked in a KFC restaurant
She needed money so badly that she could even have worked at KFC.
For clarity I made it "badly" so it doesn't sound like she needed "so much money," and I specified how that the first part of the sentence was connected to the second by using "that." I say "at KFC" rather than "a KFC restaurant" because unless someone is talking about a particular KFC, I don't hear them saying "a KFC," it's always just "KFC" and never with the word "restaurant" at the end.
The thing about sentence 1 that is slightly off as regards "could" is that it sounds almost like she had an opportunity, rather than that she would have liked an opportunity but didn't have one.
Consider: She needed the money so badly that she would have worked at a KFC.
This implies she wasn't able, but would have.
Or: If she needed the money so much, she could have worked at KFC.
This implies that getting a job at KFC is easy, and if she wanted to she could have gotten a job there.
- Jim could’ve ordered fish in the diner yesterday, but he settled for venison.
Sentence 2 is fine. It does make me wonder which diners you frequent that have both fish an venison on the menu. You must have fancy diners. [It is also possible that like most people you sometimes write diner (a restaurant) when you mean dinner (a meal).]
1a. She needed money so much, she could even work in a KFC restaurant.
1a is wrong. The first part is in the past tense, the second part, by the use of could without have is in present tense. Sentence 1 is the more correct version.
2a. Jim could order fish in the diner yesterday, but he settled for venison.
2a is wrong.
Jim could order fish at the diner. [present]
Jim could have ordered fish at the diner yesterday. [past]
Jim's choice cannot exist both in the present and in the past, unless the option continues, as in the following: Jim could have fixed the garage door yesterday, but he didn't. He could still fix it today and have it done before the party this evening.