Zak, the problem is that you are so eager to believe, so eager to find evidence that reinforces your belief that you don’t approach texts with enough skepticism to do things like look up EQUUS CONVERSIDENS to figure out when this horse appeared on this continent. At least, I’m guessing that’s what you do, and that’s what the FARMS authors bank on people like you doing, because I can’t figure out how you otherwise find this evidence so impressive.
This argument, given the ambiguous and fragmentary nature of the data in question, appears to apply equally to your own efforts to defend the status quo academic turf on the matter. Even if an intact skeleton of a modern Horse were found in its entirety in a Mesoamerican tomb, complete with its rider and a signed autograph from Mr. Ed himself, the critics would cry "fraud" and return to their comfortable space. Explaining away is the modus operandi. A creative and open mind is not. There is, of course, no reason whatever that Horses could not have existed in Mesoamerica in historic times. Its only the dearth of evidence (thus far) that presents the problem, and at least part of this can be explained by environmental factors affecting the preservation of such substances as animal bone.
I predict this: if and when the date of the last extent Horses in Mesoamerica is moved up into Book of Mormon times, the exmo's and antis will be the last to admit it, even in the face of hard evidence, and even when secular Archeology has made its peace with the data and moved on.