You would get some useful information back from bodies.
DNA evidence would conclusively prove the identity of the deceased. That may help relatives accept what happened. Insurers will sometimes require DNA evidence (if feasible) prior to paying out a large policy. And if the flight deck is still intact, DNA would confirm who was on the deck (if other forms of identification were not successful).
I don't believe the fire theory. But carboxyhemoglobin levels are increased when people are exposed to carbon monoxide (a byproduct of fire). So significantly elevated carboxyhemoglobin levels would imply a fire. Carboxyhemoglobin is pretty stable (and hence lethal).
Lactate levels may tell you something. Lactate will go up if a person is alive but in a low oxygen environment - how high it goes depends on how long the person was still alive for in a hypoxic environment, and their level of exertion (oxygen consumption) while hypoxic. Lactate will not go up in sudden traumatic death as in an auto accident. Lactate is pretty stable, and normally requires ongoing circulation to clear. It doesn't spontaneously clear after death.
Injury patters will tell you something about the forces involved, and will supplement the information you get from the wreckage.
I suspect toxicology (drug) tests will be negative in this case, but they will obviously be performed on flight crew if identified. .
If fuselage is intact and pax mostly in their seat belts, then they were dead at impact. The urge to avoid drowning is pretty strong.