I don't know if this theory is accurate or not, but I've discussed this incident a few times with a few different people; and have come to my own conclusion on part of the events of the wreck; from what I can tell the kick you describe was the probable cause, but we shouldn't point fingers on who is to blame or why. What I understand and theorize is that the force of said kick caused the 483's tender to ride up into the back of the cab. Mayer was likely on or near the tender deck at the time and would have been, while it is quite gruesome, slammed up into the roof overhang of the cab, killing him instantly. If he was on the tender deck this would also explain how he would have ended up under the engine; as humans are true ragdolls in such situations.
I believe this is also supported by the fact that if he was in the cab it would have been less likely to be thrown out and thus under the locomotive, it is a very strange place for the unfortunate fireman to end up, or so I understand from looking into other wrecks.
I'd love to hear more testimony and stories about this; I would hate to turn this into finger pointing and accusations, it would be interesting to rexamine the evidence and see if a different conclusion could be found, or a proper timeline of events and factors created.
-Peyton Smith