Admittedly the departure from controlled flight as a result of the split condition is conjecture to some extent, but given the authority the elevators have it's got to result in a serious upset very quickly.
That is a lot of conjecture on your part. Split elevators are going to result in an average of the two pitching moments and a roll moment. Since the elevators are very close to the aircraft's roll axis, any roll moment generated by the split elevators can be countered by the ailerons and spoilers which are much further away from the roll axis and therefore have a much greater moment arm, which is conjecture also, but results from the application of simple physics.
The core problem on Egyptair 990 had to be the active sabotage of the flight.