Corax's name of Corvus was the name given to him by the people who found him on Lycaeus. Its just a name. Corax however means Saviour in their tongue as we already know, and as Corax's Rebellion progressed more and more of his followers started calling him by that name over Corvus. Though those who are very close to Corax sometimes call him Corvus.
The Legions fight the battles they are best at. The Raven Guard fight with their lightning strikes against the Word Bearers on Cruciax, a small listening outpost, and against the Emperor's Children in The Perfect Fortress. The Alpha Legion fight as we expect them to, with misdirection, feint attacks and manipulating others into fighting for them. The Alpha Legion's allies in The Order of the Dragon and the Kivahr Guilds lose this battle, but the Alpha Legion win. As do the Raven Guard in a different way.
The Alpha Legion infiltrated the Raven Guard by use of extremely advanced surgery. They gathered the bodies of dead Raven Guard and surgically removed their faces and grafted them onto Alpha Legionaries that were chosen for the task. The Apothecaries remodelled the entire body of the operatives to make sure they could not be revealed as Alpha Legion. Then the Librarians, which apparently Alpharius and Omegon kept in operation despite Nikea, removed all of their memories apart from the fact that they are Alpha Legion and on a mission for the Primarch. They don't remember their names, what they look like or their whole past. The Raven Guard's memories were extracted and put into them so they can pass for them. A full Librarian scan would have easily caught them but the Raven Guard dismantled its Librarius in accordance with Nikea.
The Cabal's envoy is Athithirtir, who is described by Alpharius as a shrivelled hand with seven fingers and no thumb floating in a glass orb filled with yellow translucent gas. It speaks through a vox and throughout the novel is trying to bring Alpharius directly under the Cabal's control. It ends a bit badly for him, he isn't dead but we definitely won't be seeing him again.
Flashbacks show Deliverance/Lycaeus and Kivahr quite a lot. Deliverance itself is deceptive in appearance, Corax ordered that no damage be repaired as a tribute to those who died to bring freedom. Of course safety measures are taken but the entire planet looks quite broken, but its still deadly. Kivahr is typical of a quasi-Forge World, its filled with broken cities, polluted atmosphere and many mutants. The Ravendelve, a base where most of the action takes place, is located in the middle of a dead city that used to belong to the Guilders.
The Ravenspire doesn't make a huge appearance but we get some background on it. It used to be a prison complex and is where Corax was raised initally. It contains something called a Red Level that nobody from Deliverance ever speaks about, apparantely its where they did the worst tortures on prisoners. Corax even refuses to send a captive Word Bearer Chaplain in there, its that bad.
The Legion is restructured into four batallions bearing the name Hawks, Talons, Falcons and Raptors. The Talons are the battle-brothers of the Legion, the tactical squads. The Raptors are the recruits of the Legion and the new marines created in the novel, you'll see a good deal of them. The Hawks are the Dreadnoughts and Assault marines. And the Falcons are the bike squads, land speeders and aircraft crews.
The Emperor tells him a lot. Firstly he FINALLY! explains Chaos to him, Corax buys it straight up and understands it quickly. Now why couldn't he have bloody well told Horus the same thing?! He then tells him about the Primarch project and gives permission for Corax to use the original source material that was used to create the Primarchs to create a new generation of Astartes. He also hints that the Two Missing Primarchs were found and lost before Corax was discovered.