"could have+pp" is usually either counterfactual result in conditionals or presumption in the past. But this seems to be a factual expression like "had been able to".

Have worked or had worked?

Simple past 'worked' is the natural choice, though you can also say 'had worked'. Also, the present perfect is not just a past action before a present one: it's one where the results of the past action continue to be relevant in the present.

What does could have done mean?

used for saying that something was possible in the past, even though it did not happen.