buck converter saturating choke
The reverse recovery behaviour of the body diode is meaningless, when the diode isn't forward biased in a circuit. Simply forget about this point.
It's basically a good idea to remove the trigger transformer by changing the topology.
100 nH parasitic inductances are unsuitable, if a similar inductance is present in a 200 kHz switcher, you have to redesign the geometry, I think. I guess, you simply don't know the real circuit inductance.
Regarding FET overvoltage behaviour, please consider, that your device has already an internal protection. So you can't damage it by some overvoltage, you have to supply sufficient power to exceed the protection diode rating, if FET overvoltage would be the problem.
Did you verify, that the diode is still working, particularly is standing full reverse voltage after the FET has been damaged? The diode is much more exposed to a possible overvoltage damage, because it has no internal protection (a diode can't have it). So one possible damage scenario is breakdown of the diode and following pulse power overload of the FET. Although I would expect the diode to be damaged too, it's possibly alive at first sight.
If diode breakdown would trigger the damage process (still a guess), then a snubber at the diode could help. Reducing parasitic inductance of the commutating path can eliminate the problem.
In any case, if you don't have the tools to measure waveforms in the critical voltage/current range, it's rather difficult to clarify the issue.