For the simple past (and also positive) Could, would there possibly be any sentence that you just can't simply replace Could with Was able to or Managed to?
In general terms, such a switch should be okay. But here are two caveats:
It's not always easy to tell if what you have on your hands is a simple past without sufficient context, and as you know, you would completely change the meaning if you replaced the wrong kind of "could".
The precise meaning of "can/could" does not have perfect overlap with the notion of possibility. Sometimes it expresses permission.
— When you worked at Acme, were you given frequent smoke breaks?
— Yeah. I could step outside and have a smoke whenever I wanted.
If you replace this "could" with "was able to", you get a slightly different meaning than if you replace it with "was allowed to". Of course, there is overlap (you're able because you're allowed), but it depends on how nuanced you want your meaning to be. No doubt there are other cases where the difference betwen possibility and permission is starker.