If crews are reluctant to have cameras, why not a c heap and easy interim measure that would help with one other modification.
Put an inward looking spy hole in the door, so CC etc can see all is well.
I realise this doesn't help in this accident, so the other mod is a cabin mic that can be turned on and record to a separate track on cvr with any serious comments/concerns.
Under normal circumstances the items should be raised in a post flight report, if not they are deemed irrelevant, however in circumstances like this they could be valuable evidence.