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Who were the first people to propose a Book of Mormon Central America Theory.

What's the fallacy called when you take the argument of the intended subject (a New York Cumorah) and change the subject to something else.

Apologists will eagerly find statements of Joseph Smith speculating that some recently discovered ruin in central america was a Book of Mormon setting, as if he was the first to suggest the Limited Geography Theory. In reality, Joseph Smith believed in the hemispheric model, which included central america.

As far as a true limited geography theory focused only on mesoamerica, I don't know who first thought of it but John Sorenson popularized it.