Replace your Sons of Barabbas with the Celestial Angels for the Shattering. Sons of Barabbas are ordered instead to Prospero as you state, and Anubis is taken. The two events could be roughly close together in time, though the Prospero event should be earlier than the Shattering.
The Executioners should not be at the Shattering, unless you want them to be one of the Shattered Legions. If you do, we can replace the War Hounds with the Executioners. The only three Loyalists that should be present, discounting Dorn and his Blackened Fists, are those that are going to be broken by the battle in an attempt by Corax to put them out of commission. Since the Executioners are meant to be at Terra in force, I'd rather them not be present. It'd really lose the oomph the Shattering needs if too many Legions go through it and end up recuperating just fine to continue on.
no I'm alpharius:
I know you really want there to be a way for the twins to be actually loyal, but we need one of them to be a traitor. Alpharius is the better choice than the other Loyalists that could switch to the other side. Though you are in charge of writing it, 1000heathens has stated that his original intention was for there to be a twin on each side. Unless he changes his mind, that has to be taken into account. If you want those two Legions to go to Signus Prime, no problem. That's a great idea. But remember that one of those is either already a traitor, or will be one by the end of it. I could see the two of them sent there so that they could cancel each other out, however one ends up giving in to Chaos as a means to utterly annihilate his brother, while the other one either survives and escapes or is killed.