You do raise a valid point in the search for accuracy. The exact placement of different parts making up a helmet would indeed have varied, the main goal of the armorer being the creation of a functional object. But we do know which parts should be there and how one type differs from another. With so few helmets found, when replicating one the existing find is the Holy Grail, but there will have been variations for sure. In the Armea example the helmet depicted and dubbed "wrong" could be perfecly acceptable to any Roman soldier then, but the placement does differ from the known original. I am not really all that good in helmets, but in swords and knives also types may be recognised, but no two are the exact same. Close, maybe, but not the same.
So making your own design of a locket based on the knowledge gathered from how the existing ones were made and the way figures were portayed would IMHO be totaly OK. You cannot claim archeological provenance, of course, but you would still have a killer locket :-)
On manufacture, well, I have posted on that subject before repeatedly. It also baffles me why items cannot be made to specifications. I do suppose it has a lot to do with attitude. The production proces in India for instance is mainly not a factory type setup as we know it. Manufacturers contract out to small workshops, but then it is often again contracted out. The reading of scale drawings seems to be disregarded in that process, so one time you get what you ordered, next time things have gone horribly wrong in the quality control and the object is very "off spec". Deepeeka do have a better record, as they make larger volumes and can have their own workshop with dedicated craftsmen. But they also sell stuff they do not make themselves. I believe chainmail is one of these.
Sometimes instructions are completely ignored or things are added which are on no drawing. Like when I tried to have some blades done and suddenly fullers were added wich started 15 cm below the handle and ended 15 cm above the point, which I only learned about when I unpacked the parcel. Why? Beats me! The blades had to be scrapped. So now I forge my own
.
I hope this helps .....