Where is there any information of the traitors fleet assets during this action? The loyalist made unopposed approach to the planet and it wasn't until their " allies" appeared that fleet action commenced once the betrayal started. Further....so I am a Raven Guard captain of a battle barge and wondering why my allies are suddenly moving their fleet assets into attacking positions? Again, your fleet arrives unopposed and no one asked where is the VS or any of the other Gloriana class ships are? How do they even know Horus is on the planet? You have the greatest legion in stealth and intel gathering at your disposal, Corax should have held true to his nature and training, suspect everyone and send down his best to determine what they were up against and await the full might of 5 Legions as 3 versus 3 was not going to optimal on prepared positions....but that is another flaw in the story.
Finally, Horus, the Warmaster the ultimate General....so is the gap between him and the rest so bad no one thought to ask themselves how he was going to beat 15 other legions?
Corax and Vulkan arrived first, spending time conducting significant intel gathering missions and learning that the only way to dislodge and kill the Traitors was a ground assault. It was Ferrus that convinced his brothers that with their combined forces they could strike the initial blow and the reinforcing Legions would be on hand in a matter of hours to finish the Traitors off. Its a case of not looking the gift horse in the mouth with regards to the Traitor void assets and even if they did show up, the Loyalists had no reason to fear them as 4 more Legions were coming at their backs.
As for Horus, it was assumed that he had gone mad and from Ferrus's experience with Fulgrim, and the presence of Angron amongst Horus's ranks nobody had cause to believe otherwise. The Traitors assembled on Isstvan V had also taken a serious beating at the hands of the Loyalist on Isstvan III even excluding the fairly large number of Loyalists that were purged. Three insane generals and four battered Legions don't seem so much of a daunting task when you have 7 on your side.
Also for those confused as to how the Word Bearers could be in two places at once, the Legion was split in two after having a massive secret recruitment drive. Kor Phaeron and Erebus led the attack on Calth while Lorgar was on Isstvan. The Lorgar seen in "Know No Fear" was a Daemon masquerading as Lorgar.
As for a specific timeline of battles I believe it goes:
Isstvan III + Prospero > Isstvan V > Phall + Calth + Alaxxes Nebula + the start of Thramas + the start of the Shadow Crusade