Thanks for putting this so eloquently, I have basically the same interpretation but would phrase it way worse. I should bookmark this for reference.
Judging by Denathrius's words in the flashback, while Zovaal saw himself as the top dog, Denathrius was always influencing him, nudging him first to the open betrayal that got him chained to the Maw, then at the least providing him with the tools (the Nathrezim) to throw out hooks into the greater cosmos to take control. And had Denathrius been around in Sepulcher, he sure would've stood behind Zovaal, ready to backstab him and take control himself.
Indeed, thousands of little hooks, barely any even hit something. Taliesin explained it very well using a RL example in
this video (for a tldr of the main point, use
this timestamp)
Even among the hooks we know about, more fail than produce any real advantage. Even Argus disabling the Arbiter was definitely not planned that way, it was likely intended for the Titan Soul to be infused with way more Death magic, to directly take over the Arbiter's construct body - because the Dreadlords tried that again later in Zereth Morthis with the newly constructed Arbiter body.
Zovaal's most effective influence was rather direct: the army of Mawsworn, expanded by the Forsworn (a maybe [big maybe] initially noble but naive rebellion turned plain evil) and the Venthyr loyal to Denathrius.
The better solution would be to make, say, 3 of the "Legion" Warhorses available to players - 1 already is, albeit as a rare drop.
It's less that Xal is 5 steps ahead, it's that we are 5 steps behind, retracing the villain's plans. That's rather basic storytelling seen everywhere else, too.
Looking at the chair, it seems the green and yellow just alter the tint (in the original meaning) of the cloth parts, which are rather unichrome other than the 3D-esque shading. Any detail on armor would have to be designed without providing too much contrast, which would easily look off when the whole thing is tinted, or require complex alpha maps to not get tinted - to be fair, Ellie implies dye maps are used here, albeit of course very simple ones, to demarcate the "stripes" of the cloth and to exclude the big patch from getting tinted.
More detailed alpha maps would only make sense if we could pick&choose which part of the armor receives which tint. Unless you're aiming for that, a dye system makes no sense, just make several recolors available.