What I've studied is that in Japanese, you don't use past tenses in conditional sentences to express a lower probability (like if I were instead of if I am). I know there's a conditional formed with た form (たら), but it doesn't have anything to do with probability.
From that, I guessed that if I saw a た form followed by なら, it wouldn't mean less probability but actual past tense. For example:
should mean if I've been a musician, rather than if I were a musician. But I've heard a lot of these sentences where the actual meaning seems to be the second one.
So in what cases is it correct to use the た form before なら and what exact nuance does it give to the conditional sentence?
(EDIT) Some example sentences: