Strange, then, that the Romans would adopt both Celtic shield and Celtic ironworking and equip their legions with it.
Indeed, the Romans, like others, implemented equipment into their military standard that had much Celtic influence, but not Celtic materials or workmanship. The famous "Monteforino" helmet, for example, that so many cultures adopted. But, like the helmet, when you say it is of Celtic influence, it is a reference merely to size, shape, etc. It does not refer to its strength of material, its workmanship, or any other variations that may have been introduced (such as the Iron Boss, which was implemented by the Iberians and, then, the Romans - the boss was taken by the Romans from the Iberians, whom had introduced this variation to the Celtic scutum). So just because something has its origins somewhere, doesn't make the place of origin the cause for its many developments.

On a balancing level: yes, the barbarians get +1 armour compared to civilized armies, but that was done to increase historicity, not for game-play reasons.
Wait, if I understand many of the barbarian myths, barbarians would usually sacrifice such spoils into the rivers where they're Gods dwelt or in some other way (i.e. Battle of Teutoborg Forest and the many weapon fragments still found there to this day and accounted for in Tacitus' history).

I suspect it can be justified by the fact that barbarian warriors would often have supplemented their equipment by looting from fallen enemies and so on.
How could this ever be applied as a standard that does not cause historical, let alone basic gameplay, imbalance? Every people that ever wins a battle, to this day, gets the spoils. It is a fact of life. So why would the non-barbarian peoples not be allowed such advantage of spoils? Or is that you assume them to be so haughty as not to deem their opponents barbarian weapons good enough???

I think you are taking a too black-and-white view when it comes to barbarians. Remember that the Roman and Hellenistic states had not left their tribal history far behind. Voting in Athens and IIRC Rome still occurred on a tribal basis, and Hellenistic treaties were only valid as long both of the signers were still alive. Given that the more sophisticated Celtic tribes (the Aedui and the Sequani/Averni) had a senate and a justice system, with procedures in place to prevent abuse of power, it looks to me like they were developing along the same lines, if a century or so behind the Romans.
I understand your point - that barbarians too had civilizations - and I agree. But 100 years of difference is much more than a result of bad luck, and the scale with which each (Rome v. Gauls) is compared is too much to categorize any tribe organization with the level reached by Rome. Rome had tribes, but it began and always was a city-state, centralized by the Senate. The Gauls too had tribes, but each tribe was his own state, his own government. Yes, certain tribes were subject to others, varying from time to time, but such was the organization of Rome when it began in 758 BC. So then, either, as I said, the Gauls were a people incapable of reaching greatness, or simply were 700 years behind...

Just imagine if the United States, when declaring independence, did so as individual states, united only in that one instance of independence. Do you really think they would've achieved anything as great as they have to this day? Do you really think the many innovations brought about by this nation would've been achieved as they have been??... I don't think so.