Wow! Functional equivalence in translation, categorisation of syntactic structures and formal categorisation of verb forms all in the same breath ... That's an explanation on speed-skates.
Let me analyse that in stop-motion for a moment. In rewind perhaps:
Unfulfilled, unfulfillable or an invitation to fulfill?
In the Diodorus Siculus example, in effect, Alexander seems to be stressing that it is unfulfillable - the passage goes on to describe how they go to war with Darius - because Alexander already has his mind set (ὑπολαμβάνω is in the Luke 10:30 sense - "retort" seems to be the closest English word, but that English word carries the baggage of being a sharp or short reply, but for ὑπολαμβάνω, the shortness of speech is in the dismissive or superficial nature of the previous not following statement) on non-acceptance of the role in Darius' curia. In Chariton's story, in the account quoted there Mithridates defends himself against the charge of trying to suduce the married Callirhoe by first making a very positive statement about his life - using a very indicative aorist ἔζησα (presumably expressing the totality) and then a perfect ἔσχηκα (presumably expressing the length of time - even to the remoteness from which to the present - the amount of time that should be considered). He then uses the construction we are discussing to make an argument for his innocence based on something like common sense (culturally specific universally accepted understandings about how things should or do work) - the good governance of the king and the good effect on character of the responsibility and power - power develops good character and absolute power helps develop the good character of others too. The second half of Mithridates defence by appealing to commonsense is quite close to the way the Jesus uses the construction in our original John 14:7 (in the Byzantine version). In the second quote from Achilles Tatius it is clearly an unfulfillable conditional, as explained with the quote. In the Galatians quote, it is more like the logical argument of Mithridates - something that is not expected to be fulfilled. I think that is further reinforced by the [ἦν] δυνατόν - making it clear that he thought it wasn't possible.
There are a range of uses (ie a range of syntactic meanings) for this formula. Simply categorising is a superficial way to understand anything. Most broad-stroke grammatical categories usually contain further variegation, usually patterned.
I also had the "past" idea firmly fixed in mind when I started to understand how this formula was being used, but now, looking at a range of examples, they are formally past, rarely actually past. The range of examples quoted shows that. Now that I know that the past tense in the formula is a requirement of the think that the past tense in the formula is a requirement of the formula, rather than necessarily an indication of when the events took place or were to take place, that opens up the possibility that ἐγνώκειτε is a non-past reference word in this construction. It may be an invitation to know Jesus and hence God the Father, rather than saying in effect, "Of course you don't know the Father, because you don't know me."
The sense of ἐγνώκειτε is deoendent on a broad understanding of the formulaic syntax of the construction.
Robert, are you saying that counterfactual conditionals in Modern Greek are only constructed with the pluperfect, and that the pluperfect is the tense in both the apodosis and protasis? If I understand you correctly, I don't think the usage is so regular in Modern Greek. Perhaps we could discuss that.
The pluperfect in Modern Greek as a periprasis of the aorist of the verb έχω with an infinitive seems to have either developed in a bilingual environment along with the developments in Latin, or as part of an overall change in a post-Koine Mediterranian Sprachbund, or in a language ecology where features of a dominant language were adopted by the Greek (and Latin). In any case, the pluperfect so-called of Modern Greek is synthetic and belongs to a different verbal system, than the pluperfect of Koine Greek.
Additionally, the range of useages (syntactic meanings) that we are finding for the unfulfilled contional, might not map onto the range of uses of the Modern Greek. One of the ways forward in understanding Koine Greek grammar is to disentangle long-standing assumptions of equivalence that were held by either Byzantine grammarians or early (=Renaisance) Western ones. A lot of word has been duscussed recently about voice in that regard.
Functional equivalence between Modern and Koine Greek might not always be in paralleling the same syntactic structures. Explaining any feature of a language by the same grammatical feature in another language is in fact translation. The applicability of that approach will depend on the respective range of syntactic meaning of the structures in the languages. In other words, we need to fulfill the requirement of idiomacity in translation even when we are looking st a grammar, and the grammar is a translational grammar.