possible solution is that after deorbit burn engine could detach from stage and make its own additional deorbit "burn" with cold gas from main nozzle. so they could be separated at the point of impact.
So you're now aiming for a steeper and harsher entry with the engine. And you've just added three axis control to your engine too (or else how do you make sure the extra deorbit burn is in the right direction?).
If you wanted to make this work, it might be better to do the deorbit burn, jettison the engine, but have the stage remain active to perform additional manoeuvres- remember it already has the avionics, thrusters, and propellant. Have it enter short of the engine recovery zone, as its higher ballistic coefficient will tend to do that anyway.
I still don't know if you can get enough separation to ensure safety, though.