Modal Verbs – Difference Between ‘Could’ vs. ‘Could Have’

If you had told me, I would have used the other route.
If you had told me, I could have used the other route.

Would here denotes a certain consequence of the unreal condition: no question about it, I would have used the other route.

Could denotes a possible consequence of the unreal condition: if you had told me, I might still have taken this route but I would have had the choice of taking the other route.

If you had told^{Past Irrealis} me, I would^{Present Irrealis} use the other route.
If you tell^{Present Realis} me, I would^{Present Irrealis} use the other route.

These are not quite impossible, but the circumstances under which either would be acceptable are very unlikely to arise. Ordinarily these forms would be understood as having the tense/mode significances I have noted in superscript, and in most cases the two verbs, the one in the IF clause and the one in the THEN clause, should have the same tense and mode.^†

^† This is not always the case in conditional clauses involving logical inference; but I do not think either of these sentences can be read as inferential.

You're basically correct. But there's a lot of subtlety with could that often requires more context to resolve.

She can go to school.

This is our base reference. Nothing is unknown. She immediately has both the ability and the opportunity.

She could go to school.

There are up to three conditions not in evidence with the example. She might have the ability and/or might have the opportunity. Further, we don't know when the event is important. Generally, this statement refers to an event or decision now or sometime in the future --- but that's not necessarily the case (just the most likely case). "She could go to school. But she turned eighteen and lost the opportunity." Therefore, this statement can be used in the context of the past tense. Note that the present perfect tense is preferred when dealing with the past tense, but there are cases when it is inappropriate, such as when the discussed action is not known to be completed. "She could go to school, but was distracted by her family." Thus, the opportunity might not be lost because the distraction could go away in the future.

She could go to school last year.

This is an example of additional context that reduces the number of unknowns by resolving time. She had the ability and/or opportunity, but no longer has one or both.

She could go to school without passing the test.

This also is an example of additional context that reduces the number of unknowns by resolving ability. She had or has the opportunity or the time, but may not have or no longer has both.

She could have gone to school.

This is the present perfect tense and suggests a past event or decision has a relationship with the present. She might have had the ability and/or might have had the opportunity, but one or both was lost before today and that fact is important.

Perhaps the worst aspect of could is its use in colloquial spoken English where its use is often dependent on what other people have said. Gratefully, the majority of English speakers aren't worried about the small things, allowing the use of could to be very flexible.