Alan, with all respect, you need a cosmochemist who specializes in meteoritic minerals to answer your question. The question you are asking reduces to "can we distinguish meteoritic Fe-Ni from terrestrial Fe-Ni after it has been heated and forged." You have argued, and I think persuasively, that the crystalline structures described in sideritic meteorites may well be altered by the heating and forging processes, thereby destroying those meteoritic characteristics. Under your proposal, crystalline structures would be unreliable indicators for a meteoritic origin in a heated and forged specimen.
We must therefore look for other means to distinguish between the two options. The ratio of Fe to Ni content is one approach to try. It is said that the Ni content in meteoritic Fe-Ni complexes is higher than in terrestrial Fe-Ni by a sizeable amount.
Another approach is to look for certain elements that occur in sideritic meteorites but not in terrestrial deposits of Ni-Fe. I have offered several possible candidates. I don't know of any data looking specifically at those possibilities. Edscottite is a rare example of a crystalline structure that could distinguish meteoritic iron from other sources. Whether it would withstand heating and forging is hard to say, and it is really rare to find in meteorites (only one reported instance).
A likely answer to your question is some algorithm for elemental composition that effectively eliminates a terrestrial origin. When I say "effectively," I mean a likelihood greater than 99.9% that a particular Fe-Ni object is derived from a sideritic meteorite. That's about as close to certainty as we might get. Absolute certainty is probably an unattainable goal. Unless you see a meteorite hit the ground, recover it, and forge an Fe-Ni object from it, absolute certainty that an object is derived from a meteorite will be elusive.
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