Let us turn to whether the Chiliarchy was command of the cavalry, as previously held by Philotas.
Arrian, III 21 i calls Nabarzanes ‘the chiliarch of the cavalry which had fled with Dareios...’
Ναβαρζάνης τε, χιλιάρχης τῶν ξὺν Δαρείῳ φευγόντων ἱππέων,
And III 23 iv, simply, ‘Dareios’ chiliarch’
Ναβαρζάνης τε ὁ Δαρείου χιλιάρχης
Curtius also gives him a cavalry command at Issos III 9 i
Nabarzanes equitatu dextrum cornu tuebatur additis funditorum sagittariorumque viginti fere milibus.
This command chimes with the 30,000 cavalry and 20,000 light troops Arrian II 8 v
τῶν μὲν ἱππέων διαβιβάζειπέραν τοῦ ποταμοῦ τοῦ Πινάρου ἐς τρισμυρίους μάλιστατὸν ἀριθμὸν καὶ μετὰ τούτων τῶν ψιλῶν ἐςδισμυρίους
The numbers are, of course a nonsense but the 30,000 cavalry also appear in Kallisthenes as reported by Polybios XII 18 i, so it would seem that Curtius did find Nabarzenes command in his source and is not just giving a name he knows from elsewhere.
The identity of the ‘cavalry who fled with Darius’, of whom Nabarzanes is chiliarch, is almost certainly the ‘Royal kinsmen’; Arrian III 16 i
Δαρεῖος μὲν δὴ εὐθὺς ἐκ τῆς μάχης παρὰ τὰ ὄρητὰ Ἀρμενίων ἤλαυνεν ἐπὶ Μηδίας, καὶ ξὺν αὐτῷ οἵ τεΒάκτριοι ἱππεῖς, ὡς τότε ἐν τῇ μάχῃ ξυνετάχθησαν,ἔφευγον καὶ Περσῶν οἵ τε συγγενεῖς οἱ βασιλέως καὶτῶν μηλοφόρων καλουμένων οὐ πολλοί.
We can exclude the ‘melophoroi’ as they are infantry and the Bactrian cavalry as they were not at Issos also the syngeneis were 1,000 strong ie a chiliarchia.
There is only one piece of evidence that might suggest the Achaemenid Chiliarch was not the commander of the Kinsmen and this comes in Photios’ resume of the Persika of Ctesias
[§49] Secydianus thus became king and appointed Menostanes his azabarites . After Bagorazus returned to court, Secydianus, who cherished a long-standing enmity against him, on the pretext that he had left his father's body in Persis without his permission, ordered him to be stoned to death. The army was greatly grieved, and, although Secydianus distributed large sums amongst the soldiers, they hated him for the murder of his brother Xerxes and now for that of Bagorazus.
[§50] Secydianus, then summoned Ochus to court, who promised to present himself but failed to do so. After he had been summoned several times, he collected a large force with the obvious intention of seizing the throne. He was joined by Arbarius, commander of the cavalry, and Arxanes, satrap of Egypt. The eunuch Artoxares also came from Armenia and placed the crown on the head of Ochus against his will.
This would suggest that the ‘commander of the cavalry’ and the azabarites or Chiliarch were different offices; we cannot be sure that the cavalry command mentioned is that of the Kinsmen, indeed not having been able to track down a legible Greek version I cannot even comment on which words are used (I suspect it is simply ‘hipparch’ used in a general fashion).
So there is good evidence that the Achaemenid post Alexander took over was a cavalry command. Arrian calls Hephaistion’s command ‘the Chiliarchy of the Companion cavalry’
VII 14 10]οὔκουν οὐδὲ ἄλλον τινὰ ἔταξεν ἀντὶἩφαιστίωνος χιλίαρχον ἐπὶ τῇ ἵππῳ τῇ ἑταιρικῇ Ἀλέξανδρος, ὡς μὴ ἀπόλοιτο τὸ ὄνομα τοῦ Ἡφαιστίωνοςἐκ τῆς τάξεως, ἀλλὰ Ἡφαιστίωνός τε ἡ χιλιαρχiα ἐκαλεῖτο καὶ τὸ σημεῖον αὐτῆς ἡγεῖτο τὸ ἐξ Ἡφαιστίωνος πεποιημένον.
In Photius’ summary of Arrian’s Events after Alexander Kassander is
chiliarch of the cavalry
And Perdikkas is initially
that Perdiccas should be chiliarch of the troops which had been under the command of Hephaestion
Photius’ summary of Dexippos has
Perdiccas obtained the chiliarchy of Hephaestion, the highest dignity amongst the Macedonians
Diodoros XVIII 3 iv
4 He placed Seleucus in command of the cavalry of the Companions, a most distinguished office; for Hephaestion commanded them first, Perdiccas after him, and third the above-named Seleucus.
[4]Σέλευκον δ᾽ ἔταξεν ἐπὶ τὴν ἱππαρχίαν τῶν ἑταίρων, οὖσαν ἐπιφανεστάτην:ταύτης γὰρ Ἡφαιστίων πρῶτος μὲν ἡγήσατο,μετὰ δὲ τοῦτον Περδίκκας, τρίτος δ᾽ ὁ προειρημένος Σέλευκος.
The only offices we know of shared by Perdikkas and Hephaistion are the Chiliarchy and an ordinary hipparchy which would not be described as ἐπὶ τὴν ἱππαρχίαν τῶν ἑταίρων, οὖσαν ἐπιφανεστάτην; we can note too the similarity of the description of this post here as ‘ousan epiphanestataten’ and in Dexippos apud Photios as ‘protiston times telos’ honour of the first order. A typically innacurate paraphrase (it should apply to the prostasia/epimeleteia rather than the chiliarchia) cf the description of Antipater’s area of authority where Dexippos allots him power over ‘... the whole of Macedonia, Greece, Illyria, the country of the Triballi and the Agrianes’ Whereas reference to the summary of Arrian gives ‘the country beyond Thrace, as far as the Illyrians, Triballians. and Agrianians, Macedonia itself, and Epirus as far as the Ceraunian mountains, together with the whole of Greece, to Craterus and Antipater’ Dexippos seems to have confused the limits of Antipater’s puview with its extent, The fact that Photius can get this right when summarising Arrian leads one to suspect the error was Dexippos’.
The reason for thinking Arrian conflated the offices of Hephaistion is not based on anything in the sources but on the mistaken belief that the ‘hazarapatis’ functioned as a sort of Ottoman Grand Vizier; the analogy is false. The only attested role for him is controlling access to the Great King, Cornelius Nepos ‘Conon’ iii 2
Conon, being sent by Pharnabazus to the king to assure him of his guilt, went in the first place, on his arrival (after the manner of the Persians), to Tithraustes, the captain of the guard (chiliarchum), who held the second place in the empire, and signified that he wished to speak to the king; for no one is admitted without this ceremony.
2 Huius accusandi gratia Conon a Pharnabazo ad regem missus, posteaquam venit, primum ex more Persarum ad chiliarchum, qui secundum gradum imperii tenebat, Tithrausten, accessit seque ostendit cum rege colloqui velle. Nemo enim sine hoc admittitur.
Or Plutarch ‘Themistocles’
27 Now Thucydides25 and Charon of Lampsacus relate that Xerxes was dead, and that it was his son Artaxerxes with whom Themistocles had his interview; but Ephorus and Dinon and Clitarchus and Heracleides and yet more besides have it that it was Xerxes to whom he came. With the chronological data Thucydides seems to me more in accord, although these are by no means securely established. 2 Be that as it may, Themistocles, thus at the threshold of the dreadful ordeal, had audience first with Artabanus the Chiliarch, or Grand Vizier, and said that he was a Hellene, and that he desired to have an audience with the King on matters which were of the highest importance and for which the monarch entertained the most lively concern. Whereupon the Chiliarch replied: "O Stranger, men's customs differ; different people honour different practices; but all honour the exaltation and maintenance of their own peculiar ways. 3 Now you Hellenes are said to admire liberty and equality above all things; but in our eyes, among many fair customs, this is the fairest of all, to honour the King, and to pay obeisance to him as the image of that god who is the preserver of all things. If, then, thou approvest our practice and wilt pay obeisance, it is in thy power to behold and address the King; but if thou art otherwise minded, it will be needful for thee to employ messengers to him in thy stead, for it is not a custom of this country that the King give ear to a man who has not paid him obeisance." 4 When Themistocles heard this, he said to him: "Nay, but I am come, Artabanus, to augment the King's fame and power, and I will not only myself observe your customs, since such is the pleasure of the god who exalts the Persians, but I will induce more men than do so now to pay obeisance to the King. Therefore let this matter by no means stand in the way of the words I wish to speak to him." 5 "And what Hellene," said Artabanus, "shall I say thou art who hast thus come? Verily, thou dost not seem to be a man of ordinary understanding." And Themistocles said: "This, Artabanus, no one may learn before the King."
So indeed Phanias says, and Eratosthenes, in his book "On Wealth," adds the statement that it was through a woman of Eretria, whom the Chiliarch had to wife, that Themistocles obtained interview and conference with him.
Tithraustes is called the ‘second in the kingdom’, but he is the only Achaemenid Chiliarch to be called this and Ctesias cites three or more people with influence over the King for each reign most of whom are clearly not the Chiliarch.
Since the Chiliarchy is the cavalry command we can say something further about the date of Hephaistion’s assumption of it. It cannot have occurred during the life of Kleitos as he and Hephaistion shared the command as hipparchs. Nor is any overall commander mentioned, as Philotas was, during the narrative of the Indian campaign. The occasion associated with medism and cavalry reform is Opis/Susa.
I wish I could claim to be the only person seeing things this way but Alexander Meeus, Pierre Briant, Josef Wischoefer among others all beat me to it. Critical reading of the sources and their modern interpreters is a duty rather than a prerogative; unless one just wants to pass ones time in the grassland quietly away.