Just for reference: the subjunctive is a "mood" and not a tense, just as the indicative or conditional. That said, one would think that the subjunctive mood should exist in all tenses, which, however, it doesn't. There is only the subjunctive present and past tense in common use. And of those two, the past subjunctive is only used in order to emphasize that one action started after another action has already finished. In pretty much all other cases you use the subjunctive present. So, the past subjunctive isn't really used to say that something happened in the past but to emphasize the temporal relation between two events in the past.

in the phrase "...bien que je ne sois pas d'accord," I would really like a precise explanation of why sois (present) is correct rather than the past (ai ete). Telling me it's a mood and not a tense does not help in any way.

Did you read the entire post? Because if you did, the answer is spelled out right there.