Glued Linen Armour- a simple test

[Paralus]

I agree that the armour is not stated to be that of Darius. And, as you state, the ancdote is not mentioned in the other extant source material. Either Ptolemy/Aristobulus and the Vulgate did not record this or Plutarch found it in a source unknown to either or he delicately "inserted" it as flavour. He is very detailed in his flavouring though and so one supects that there is some reality to it and it has slipped the other two traditions in some fashion.

Given that what Plutarch retails is fact, I can only see this cuirass as coming into Alexander's hands with the rest "of the spoils" taken after Issos. These spoils are clearly the Great King's advance camp at Damascus and his Ipsoss "headquarters" which provided much cash readies, Darius' family and - a fortiori - his possessions. Arrian gives an inkling after Gaugamela when he desribes Alexander's windfall at Arbela:

...his [Darius'] treasure, however, and all his valuables fell inot Alexander's hands, including his chariot, shield and bow...

This tallies with the description following Issos where the "headquarters" of Darius was stormed and taken as well as the main advance camp at Damascus where "all the paraphenalia" of Darius was stored. Plutarch himself adds rather more decadent details in his description post Issos:

He found his Macedonians carrying off the wealth from the camp of the Barbarians, and the wealth was of surpassing abundance, although its owners had come to the battle in light marching order and had left most of their baggage in Damascus; he found, too, that his men had picked out for him the tent of Dareius, which was full to overflowing with gorgeous servitors and furniture, and many treasures.

There is little doubt that if the spoils included war chariots, shields and bows it likely also included armour. I doubt that Alexander who, post Issos, clearly declared his "legitimate" status as the real king of Asia would don the armour of one of the king's "kinsmen", his lessers. Alexander, if he wore Persian armour, is much more likely to have deported himself in the great King's armour. Not only did he have the royal women he now took the field against him in his own armour.

All given, of course, it was a good fit...