Meaning of た form + なら


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:

> もしも僕がミュージシャンだったなら

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:

> もしも僕が名医だったなら、親父の病気は僕が治す。

> もしも僕が王様だったなら、嫌いなやつは全員消えてもらう。

> もしも僕がミュージシャンだったなら、言葉にならない言葉を紡ぐ。

