\"Celtic\" military technology and the Romans

[Renatus]

The original text:

quos lamminarum circuli tenues apti corporis flexibus ambiebant per omnia membra diducti ut,

I think that this is reasonably translated (with only the words relative to the point in question included) as "circular plates surrounding the body and every limb".

No, it isn't. You are treating corporis and membra as if they were the objects of ambiebant, which they plainly are not. The object of the verb is quos, referring back to cataphracti equites. Corporis is in the genitive, not the accusative, and forms part of the subordinate clause, agreeing with circuli, "apti corporis flexibus", literally 'fitted to the bendings of the body'. 'Body' here means the body as a whole, not simply the trunk; the 'bendings' are the elbows and knees. Membra is part of the subordinate clause "per omnia membra diducti", literally 'divided through all the limbs', in which diducti also agrees with circuli. Ammianus goes on to state that in whichever way necessity caused the joints (artus) to move, the armour followed suit. The passage has to be read as a whole. He is evidently referring to armour for the limbs only.