The subjonctif plus que parfait is composed of the subjonctif imparfait (eussions) followed by a participe passé (inventé). I don’t know about the birth of this tense, but I can tell you that the subjonctif imparfait (imperfect subjunctive) comes from the latin subjcontif plus que parfait (pluperfect subjunctive). It inheritated the weak verbal base, followed by a thematif vowel – accentued for P1-2-3-6- (a / i / u) then the flexional endings (e, es, t, ions, iez, ent) -accentued for P4-5.

For example, the latin amavissem (first person of the pluperfect subjunctive) changed to amassem (late latin), then lost the final -m (imitating the lost of the final -m of the nouns). The example is quite difficult because the verbe aimer used to have two bases aim- and am- depending on where was the accent / intonation. Only aim- remained at the end of the Middle Ages / Renaissance.

But the French plusperfect subjunctive uses the auxiliary avoir which is even more complicated. It goes a bit like that = habuissem > abwissem > awwisse > owwisse > changed to resemble the indicatif past tense = eusse.

That’s all I have, hope that helps you a bit.