And frankly it is an untenable hypothesis given the state of the evidence, no? I mean, unless theology had driven them (hypothetical Nephites) to imagine a history that has little or no connection with what actually happened, how could it be possible?
This is exactly correct. Defenders of the faith who do a little victory dance every time a new discovery has been made in ancient Mesoamerica, or a scholar states that there is much discovery yet to be done, are the same defenders who create the strawman that critics claim "all the discovering has been done". Complete, utter, nonsensical bullsh*t.
What is true that enough discovering has been done for scholars to have a good idea of what the ancient Mesoamerica culture was like. Any future discovery that could possibly validate the Book of Mormon would have to overturn practically every bit of knowledge scholars accept today.
It is a more fruitful avenue for apologists to explore your second hypothesis, in my opinion. It's my favorite "pimply Maya teenager sitting in his mother's basement writing fiction that became the Book of Mormon" theory. Apologists already accept that many portions of the Book of Mormon have little connection to what actually happened due to "translation issues". They just have to swallow the bitter pill and take it one step further.