Could / Be able to / Could be able to

Question

1. I mean, we'd love to leave people pristine so we could be able to study them but it's not ethical.
2. I mean, we'd love to leave people pristine so we are able to study them but it's not ethical.
3. I mean, we'd love to leave people pristine so we could study them but it's not ethical.

It always baffles me when it comes to the usage of Could. I know Could is used for general ability whereas Be able to for a specific occasion but this doesn't do much good to me most of times.(Don't they number 2 and 3 both mean 'a possible occasion in the present or future'?) I've looked up a good many related Q&As in this site but they were all the same to my non-native eyes and not a single question was about the put-together one, "Could be able to" at that. Could anyone be nice and parse those threes above step by step and clarify what's the differences?

PS: The original sentence, which I got from COCA, is the first one. (could be able to)

Answer 1

Sometimes you want to express "the ability to do something" in the conditional mood. Sentence 2 ("I mean, we'd love to leave people pristine so we are able to study them but it's not ethical.") is not completely grammatical to my eyes/ears. "we'd love" and "are able" should match in mood/tense. So either "would be able to" or "could be able to" would work as a counterpart to "we'd love to".

"Could" by itself doesn't seem to work (sentence 3) and I'm not sure I can explain why. I think "could study them" suggests something like "we would study them right away given the right circumstances," while "could be able to study them" is more like "we would have the tools and the knowledge to study them, but maybe wouldn't study them - the ability is enough." Something like that - there is a subtle difference.