I realize we’re on BoingBoing so we both need to oversimplify our arguments, but we owe other readers accurate physics.
Notice the plummeting highway death rates in the developed world over the past 40 years. Did the cars get dramatically heavier? The cars got safer, but not in a way captured in the physics you’ve described thus far.
Momentum transfer between the vehicles is not what determines the degree bodily harm (except in Phys 1). The relevant physical quantity is energy dissipation within each human’s body.
The naive, incomplete physics you presented above do suggest the occupant of the lighter vehicle will sustain great injury. This naive physics is adequate for, say, a '54 Dodge Dart colliding head-on with '57 Cadillac El Dorado at highway speed. The occupants of both cars would probably be dead, but the folks in the Dodge Dart would definitely be dead-er.
In the post that started our discussion, our protagonist was thrown clear. That is, the collision wasn’t inelastic. Thus, your analysis is incomplete. In particular, transforming our protagonist’s experience from a partially elastic collision with fragmentation – a complicated problem to analyze – into an inelastic collision ( by putting him in a comparably sized car) might have led to a worse outcome.
I agree that heavier does genereally mean more momentum transfer to the lighter object, but in the context of modern vehicles heavier does necessarily not mean safer.
America’s highways are more lethal than necessary in no small part because so many do not understand this subtle but vital distinction.