The real story is that Strephon's greatest secret, his insatiable chocholicism, killed him when he ate too much the night before Dulinor's arrival (a Theobromine/Sugar overdose; his doctors kept warning him to avoid the stuff, but he just couldn't stay away). (Alternatively, you may insert you favorite depraved scandal, the nature of which I'll leave as an exercise for the reader, not to be detailed here . . . please.)
The Imperial Guard commanders concocted a scheme to frame Dulinor for the assassination of Strephon.
They used one of Strephon's robotic doubles to attend the ceremony with specific instructions loaded. Then, they forged the entire famed holographic scene broadcast to the public at the same moment that other events were actually occuring in the Octagon.
What really happened is that the robotic double of Strephon ordered Dulinor beheaded. When the Ilelish Guard refused to comply, the Marine Guard rushed in and a firefight commenced. Dulinor and elements of his domain guard fought their way through the palace in a complicated series of manuevers, eventually allowing Dulinor to flee. The Imperial Guard commanders allowed him to get to his starship and escape, thinking to follow him shortly and "contain" the madman who had just assassinated the Emperor.
Unfortunately, things didn't quite turn out like they had planned . . .