The least reliable thing is eyewitness reports. I was a juror on an armed robbery trial ten years ago. 5 eyewitnesses and all 5 saw an entirely different event. All of them contradicted each other. Luckily we had DNA from a hat that fell off the robber's head at the restaurant.
We don't retrofit DEF. The older units use a heat based burn off. The newer units come from the factory equipped with DEF units, DEF is a chemical based cleansing that is much cooler than the older units.
NTSB has already said that there's no evidence in the median to support the tractor being on fire.
I won't speculate on what happened, just waiting to see what the experts report.
ST