You clearly stated the following which is wrong.
"Well, the damage to his voice box could only come from external physical abuse. In other words, someone had to compress his larynx for that to happen."
Supposedly, his fall after breaking his neck did that.
As for this banging argument and requiring something else.
He is thrashing about and slips.
That is still a result of his banging.
He is ramming/banging his head agaisnt the rear door and slams it into the bolt, shifting and concentrating the force breaking his neck.
Still the result of his ramming/banging.
Batts said another man who was in the van during the tail end of Gray's ride told investigators that Gray was "was still moving around, that he was kicking and making noises" up until the van arrived at the station. Batts said the man also said the driver did not speed, make sudden stops of "drive erratically."
Freddie Gray death: Baltimore police prisoner transport under scrutiny - World - CBC News
That is the evidence, not something else.
The report clearly stated "and for some reason, went flying in the van".
Some reason! And as the evidence says the van was not being driven erratically, speeding or making sudden stops, the focus goes right back Gray's actions.
And this nonsense about believing another thug is just that, nonsense.
For some reason people do not understand that there needs to be something there to question any persons word.
Simple speculation is not sufficient.
As it stands, it is evidence, and it happens to coincide with the physical evidence.