I assume we have conditionnel passé deuxième forme in the following:
Si fâcheux que fussent ces contretemps, en vain les accuserais-je. Quand bien même tout nous eût secondés, nous eussions inventé notre gêne. Mais qu’Alissa, elle aussi, le sentît, rien ne pouvait me désoler davantage. Voici la lettre que, sitôt de retour à Paris, je reçus :
QUESTION
What are the historical stages by which the form came to be known as 'conditionnel passé deuxième forme' as well as 'subjonctif plus-que-parfait'?
BACKGROUND
Just to illustrate what I mean by 'historical stages' I am going to make up the following story. I don't mean that this is what happened.
Latin did not have a conditional and used subjunctive to express a counterfactual statement.
In early French conditional was developed as a tense within indicative, e.g. in case of conditionnel présent to mean the future of (from the perspective of) a past. The future of a past can be seen this:
Pourquoi cette gêne, ce sentiment de fausse position, cette paralysie, ce mutisme, quand nous avons tout à nous dire ? Le premier jour de ton retour j’étais heureuse de ce silence même, parce que je croyais qu’il se dissiperait, que tu me dirais des choses merveilleuses ; tu ne pouvais partir auparavant.
Conditional then drew to itself a second function, that of expressing counterfactuals.
Conditional became the dominant means of expressing counterfactuals.
Subjonctif plus-que-parfait, now chiefly used for purposes other than the expression of a counterfactual, acquired a second name 'conditionnel passé deuxième forme,' by which it came to be known on those rare occasions when it did express a counterfactual.
The quoted passages are from chapter 6 of La porte étroite by André Gide.
This and this earlier post may be relevant.
It would be great if the answer indicated roughly when each stage in the development took place. Please, however, do not feel that an answer must be complete. Any helpful information will be appreciated. Thanks.