Was the alleged EMS weapons qualified? Would they know the casings well enough to make that judgment? In court, and coming from a nonexpert witness, that would probably be struck down as conclusory opinion rather than admissible statement of observed fact. The same caution of course applies to the new unnamed witness.
Another wrinkle to consider. I am not an expert on guns either, but I am told a 9mm upper can be set on a Bushmaster lower. This has certain advantages for close range shooting (in which, BTW, I would personally prefer a pistol), such as less recoil, less overpenetration, etc. So it is at least a theoretical possibility both things are true, 9mm casings on site and a Bushmaster used, if the Bushmaster was custom.
And if it was custom, why not modded to be full auto? There are kits for that, which a prepper mom and true gun enthusiast would likely know about. And if full auto, it wouldn’t work as a wedge issue for a gun ban, because while Heller’s “unusual and dangerous” category might well apply to the custom weapon, that test wouldn’t have any traction at all on the standard, “commonly used” semi-auto weapon, and thus Lanza’s actual weapon as used would not work as a symbol for the gun control push.
And there remains the question of fear. If multiple unnamed witnesses are in fact afraid, what is the basis for that fear?