So the Emperor needs to be really careful when attacking the VS, because he's not getting out and all the chaos troops on the landing pads are potential reinforcements for Horus. Very dangerous.
I'm gonna just outright disagree with this assessment. If the Imperials get Boarding Action off, Horus is dead, nothing to reinforce. Granted, it takes a little groundwork, namely Assemble off the strat map (and probably BA off the strat map too), but Assemble + BA (just 2 initiative needed for these two cards) + Assault/Onslaught from hand = gg, Imperials win. And that's IMO the only reason and time to do a Boarding Action, straight out win, otherwise it's a waste of actions and troops.
You'll be bringing 3x Custodes, any number of Imperial Fists + Blood Angels and Emperor plus both Primarchs, meaning every Imperial unit counts as rank IV thanks to Dorn (drawing in excess of 20 cards) and with the Emperor's ability, you'll cycle through the combat deck in enough numbers to get the one card you need: Driven Before the Storm, which has a combat effect that cannot be cancelled of routing two units of lower rank than one of your units, meaning two Chaos Space Marines get eliminated (you're rank IV, they are III plus they cannot retreat, no available area), leaving Horus with just a single unit to hold off two more active Imperial iterations, with Sanguinius + BAs effectively cancelling out the fortifications of the Command Center. Traitors could protect Horus in scenarios II-VI by putting 3 Titans in the Command Center, but that really hurts their offense on the ground, probably too much.