Does the use of "could" in past ability always imply the ability was used? Do I always have to use "could have" for an event that was possible but did not happen?

I apologise for writing such a long post but I have been tackling this question for about 2 weeks and unfortunately I could not find a logical answer that made me go "aha! I've got it now!". This question seems to have a very subtle point that I've been constantly failing to discern, so it could even be a highly controversial philosophical question concerning linguistics. Anyway, here we go

First of all, I know "could" and "could have" have lots of different meanings depending on the context, but the meaning I would like to ask about is past ability, as in "she could speak 4 languages when she was 12".

This said, I have always made the distinction between the performance of the ability and the possession of the ability. So to speak, to say "I can run 100 kilometers" does not NECESSARILY mean "I DO run 100 kilometers", or "I HAVE RUN 100 kilometers once before, even though most of the time, the latter is implied when someone says the former.

In this respect I completely understand why we say " I have never run 100 meters but I could run", because we reason there must be a cause, a reason behind his not having run despite having the ability. Saying could rather than can adds a subtle "if" at the end of the sentence that does not elucidate why he has never used his ability, but still indicates his omission, as if to say "I have never run 100 meters but I could (if I fancied so I have the ability but I don't use it)

Accordingly, when we apply this to the past events, I again understand it sounds much better to say "10 years ago, I had never run 100 meters but I could have", rather than saying "but I could". Saying "could have" in this context sounds better, more idiomatic, whatever, I completely agree and I also use could have in this regard but the question I would like to ask is whether it constitutes a grammatical error or a logical error, or any other type of error that makes using could in the sentence "I had never run 100 meters but I could", apart from sounding worse or bizarre, to use could instead of could have in the sentence above

I asked this question many times and the typical answer I got was "you cannot be sure of your ability unless you have at least once tried to do it". However, this makes no sense to me because so far I have never counted to 1000 but I can do it right now. Or, I have never jumped 30 times in a row but I can do it right now. I am 100% sure I can do both of these things, so why do I have to have used my ability at least once to entitled to say "I can do it"? And the same way, why is it said that I had to have used my ability at least once to know whether I could do it? I had never counted to 1000 until I was 15 and I am sure I had the ability, that is, I could do it, but I did not want to.

Briefly, is it an absolute error to construct a sentence that bares the logic "I could but I did not". Do I always have to say "I could have but I did not" insofar as a past ability is concerned?