Abbadon needs a reason to be pissed?
...you know... that actually might explain the currrent state of affairs nicely. Emperor already dead and the High Lords keeping things hush hush. Of course the 'soul bindng' would have to be explained, but considering how long the Throne has been in contact with the emperor it's bound to leave imprints, which might explain things.
I always though the soul binding allowed for astropaths to reach eachother easier and over longer distances than their own innate power would account for as well as provide a bit of an added defense against daemonic possession.
Assuming your theory is correct, than the Golden Throne itself could be the mechanism by which Astropaths are collectively bound -- the corpse of the Emperor merely for show. Therefore the Astronomican could, theoretically speaking, be nothing more than one Astropath looking threw magnified lenses at the soul-light of another Astropath.
The other hole in my theory is that to my knowledge the Emperor's ascension was nessecary in the first place to prevent the Warp from spilling in through the Human Webway that he was creating.
The other, which I may have just accounted for, is the fact that millions of psykers have supposedly been sacrfiiced to Him over the last 10,000 years. Assuming the Golden Throne is simply some vastly ancient machine, then perhaps the corpse of the Emperor is again just a show and the psyker's souls are simply sucked into the machine to feed it so that he will forever hold the warp from spilling into Terra.
See, right there, GW could take that idea and expand the story, continue it. The Emperor died X thousands of years ago and the High Lords have just used him to maintain control. The Astronomican is a complex lie and the Golden Throne, not the Emperor, prevents the Aether from spilling into the Materium.
Hell the next 10,000 years could be spent with the Emperor ressurecting upon some Emperor-forsaken world and slowly, over thousands of years, prepare his path to ascension once again.