He should have been stripped of military command for practicing sorcery, then made the Chief Imperial Diplomat. That ways, his thirst of knowledge can be put to good use and his sorcery can be limited to using his prescience and psychically-enhanced charisma. Just imagine what the Imperium would be like if it brought Interex, Diasporex, and Aurelian Technocracy into its fold without any violence not to mention forge an alliance with the Craftworlders and Harlequins to help gain access to the Webway (and Exterminatus the crap out of Shaadom and Commorragh).
Some other traitor primarchs could have also been kept loyal by giving them civilian posts, as Lorgar would make a great Propaganda Chief, Fulgrim a great Chief of Manufacturing and Logistics, and Perturabo the High Fabricator.
The blood ravens know neither their founding chapter nor primarch.
While it is very likely, there are no official sources stating they are offsprings of the thousand sons.
So what? McNeill dropped some hints as do several other authors.
But there is one big thing which states that Magnus isneither the primarch of the BR nor are they a successor chapter of the Thousand Sons.
It is also likely that the Blood Angels, Raven Guard or the Dark Angels are their parental legion. Or what about the Imperial Fists?
They have several relics of them...
And to cut this short: Untill the day, Games Workshop drops the bomb about Magnus being the primarch, these are all rumors.
The Blood Ravens could as well be the second or eleventh legion, who knows? No one, thats the point.
GW love their speculation thing and now... back to reading Horus Heresy fiction... and please... no C.S. Goto... that guy is the worst BL ever hired...