Okay so,

Firstly, WELCOME! This is one hell of a hobby and I hope you enjoy your stay.

First, Horus' head tubes.

No idea. I'd hazard a guess that Horus needs them to deliver meds or stims into his brain and body. By Heresy's end, he had received several mear fatal wounds, from Davin, from Lemun Russ himself on the ill-fated preemptive strike that knocked the Vulka Fenryka out of the Siege of Terra, and he was a vessel for the Chaos Gods. Abaddon he is not. Horus actually pledged himself to the Chaos gods, and likely suffered corruption as a result- a need for stims from Slannesh, the pumping of constant adrenaline from Khorne, the desieses of Nurgle and lest we forget the mutation clusterfunk that is Tzeentch. -----TL;DR I don't know for sure but it's probably a meds feed

Second, Emperor's Perpetualism I'll handle this one statement at a time since you asked quite a lot, which I don't blame you aha.