Surely his quiver has internal subdivisions (at least three) allowing him to subgroup arrows. Then, at 1:25 you can see that his arrows have two-feather fletching, which determines the orientation of the nock, since the feathers must sit horizontally when the arrows are nocked. When grabbing multiple arrows in one hand, the feathers just sit in parallel layers on top of each other, correctly orienting their nocks. That’s why he can nock four at the same time so fast a few seconds earlier. He has a bit of a harder time with five, but I think it could be achieved with practice, and maybe a few extra adjustments in equipment. In addition to all that I mentioned before, the lethality of an arrow “has very little to do with the KE it possesses“, but that it “comes from broadhead design and durability, arrow design and durability, and the penetrating power (momentum) it carries” (https://cervicide.com/arrow-speed-vs-weight-which-matters). And of course, composite warbows would shoot arrows much harder than on these videos. Still, I’m not convinced that this was indeed what the Sasanians were doing, instead of just holding five arrows to shoot them in fast sequence, but at least it would seem more plausible than one initially would think. Certainly “physically possible”, hard to say if "physically makes sense".

How does he manage to pull three arrows from his quiver in a single motion, and how does he manage to nock three arrows onto the string at once in a single motion? I'm genuinely curious because it seems to me that it's not that easy for other archers:  

When cutscene mode is enabled, then no, that's kinda the whole point of cutscene mode. When it's not enabled, on other hand, you can, but only to some extent since the HUD (selection panels etc.) is hidden. You can give gather and attack orders, for example, but not train units or research techs. Admittedly, this is a limitation, but it's the best solution I managed come up with, the in-game HUD itself is already pretty cramped in my opinion.   Yeah, as shown in the second half of the demo video, this is possible. Currently, the unit's displayed name (e.g. "Champion Infantry Pikeman") is used in combination with its in-game portrait. Would be possible, though, to extend it to allow customising those too.

On these cuirasses, I think the armor would have gone a little higher over the clavicles. I know the existing ones aren't really consistent on this or are consistently wrong. 

Ah yes, the current body mesh does have some odd distortions.