1. The shamans created the Emperor because they were being killed off one by one by daemons while waiting for reincarnation. Chaos already existed back then.

2. Per the Black Crusade Core Rulebook there have been several chaos-gods in the past. Khorne, Tzeentch, Slaanesh and Nurgle are just the most recent ones.

3. He told Magnus about his webway-project but warned him to keep it a secret and not to meddle.

4. The Chaos Gods are in the warp and therefore timeless. They are a part of the Warp itself. They don't need humans and there are xenos-species that worship Chaos.
IMO, without the Imperium, those species and the Orks would rule the galaxy.

5. There was no time to wait the Battle of Terra out. The Imperium was losing and the traitor-forces were already inside the imperial palace, gaining more and more ground. Time was running out for both sides.
Read Collected Visions or the new Horus Heresy Artbook.

6. The Golden Throne is a piece of archaeotech or xenos-tech. It's purpose is to control the Webway and to keep the user alive. The Emperor is only sitting on the Golden Throne because these are the only systems that can keep him alive.

7. Per Talon of Horus, the psychic energies of the Astronomican and of Chaos are anathema.