Quick one before I duck out - the Emperor knew of Horus' betrayal before Magnus' message by regular means: thousands of Astropathic communications going "OMG Horus has gone crazy and massacred half his d00dz". So Magnus' warning was unnecessary in the most basic sense and achieved nothing.
Evil Horus telling Russ to kill Magnus is gone, as the sacking of Prospero occurs roughly simultaneously with the Drop Site Massacre (I'm going by 'first hand sources' - GW books, not wikis).

Also big thanks for all those suggestions - more later