Creamie, that is entirely possible.
The backfires people refer to are actually AFTER-FIRES in the exhaust, and the very plausible explanation is this. And remember the witness report I have is the plane pitched violently upwards as these sounds changed/happened.
Seat slides back on rail, or perhaps as has been seen before, the seat is not located on all points on the rail and it has slid back due to not being fully latched in.
Pilot has fist full of control column and throttle, so with rapid rearward movement, the natural tendency is to hang on and pull forward. This gives rise to a rapid pitch up and a rapid throttle closure.
The pitch up followed by stall spin is easy to imagine.
The rapid closing of throttle, with a TCM IO550 which this plane had, now has a fuel pump turning at 2700RPM and full rich and all of a sudden a rapid closure of throttle and air, but fuel still being sprayed into the intake port, overly rich mixture does not produce power and is expelled and igniting in the exhaust.
Then allegedly there was a recovery of engine (may be some conjecture here but I am not 100% sure) so an attempt to restore power perhaps, but it is all too late.
Now, do not take this to the bank as a "case closed" explanation. But the reports I have had, along with known things that can and do happen make this a highly likely scenario.
I am sure time will tell. If there was a genuine engine failure, this should come out in the subsequent inspection.