(i.e., before he ever had a speaking part.)
Is this cannon? He now communicates via physical means?
Yes. When you say "now" it implies it's the only way he communicates. He uses the full gamut of communication skills. Verbal (usually written as though GOD is talking at you), dreams, telepathy, projection.....
I think the one instance I liked the least was when Dorn and Malcador were in a room in the Imperial Palace arguing and then the Emperor just kind of materializes from a wall and
Starts talking in bold and italics, for puny unformatted text cannot capture his sheer dynamism.The Space Marines never could have taken down the Emperor. Hell, the Horus Heresy as a whole almost failed. Only that critical point of using Horus emotionally against the Emperor allowed him to be struck down. And besides that, the Space Marines never would have had the corruption rates they did if not for the Primarchs to convince them. The HH with every Legion having the defection rates of the loyal ones would not be too impressive.
1,000,000 Astartes could over run the Imperial palace. If HH 40k has proven anything, it's that no one is safe. No one is invulnerable. Even Vulkan, even the Emperor. I get this image of a mountain on Space Marine dead piled on top of the Emperor as he just kind of stabs and psychic blasts his way to the top, only to have more pile on.
It's 40k's obsession with epic duels that leads us to think you just can't kill some people. Before it turns right around and show that enough bolts and/or blades will get the job done on just about anyone. I'm pretty sure if you decapitated the Emperor he might still be around in an energy manifestation or spirit form but his time on the physical plane would more or less be done just like anyone else. He's a Perpetual just like Grammaticus but he doesn't have Eldar constantly raising him from the dead.