That's easily doable, the entire scene is a background, and when two parts start to assembling, those two parts becomes sprites. The effect is similar to how all the devils works.
Weren't you talking about the first fortress boss in Rockman9? That's the faced turrets with the spike balls. Upon reviewing the actual battle, I notice there's definitely one palette too many in use once the red charging ones make their appearence, but the amount of sprites on screen shouldn't present any real issues beyond some minor flickering. With some simple alternations and tricks, the battle is possible to recreate.
As for the Twin Devil. That one is positively insane to pull off, but definitely somewhat feasible. It's just that the amount of flickering and slowdown cannot be determined without programming it into an actual NES.
You're in a minority of people that want to see lazy edits instead of original works. All made possible by gripping to the idea that if the NES couldn't do it, then the game shouldn't try. Which is absurd. You can still have an "8bit artstyle" without clinging to archaic hardware limitations.
You're thinking of this in reverse. By working within those "archaic" hardware limitations, you're forced to employ creative solutions to common game design problems which defined the very core aesthetics of all games on the platform. You have to know what the system can do, then bring out its potential to the max. Blatantly ignoring that doesn't make for better sprites, it just needlessly clutters with detail and color and makes things stands out like a sore thumb.
If you're going to defy the limitations that define the NES, why not go for 16 color sprites, or full-blown HD for that matter?
I dunno, Zan... Sounds more like excuses to support your argument IMO. "Oh it's hard to tell if the NES could do it or not..."
Well, it's not that the NES couldn't do it. It's just a given that Forte with a more extensive white overlay than Rock (spread over his entire body compared to just his face) would sooner promote flickering and slowdown. This is not something that can be helped, but it's certainly something that needs to be be minimized; especially since the limits are further strained by him having rapid fire capabilities.
That said, they totally could have made a better sprite, but over-coloring as in the previous fanmade one, isn't the way to go. This new rendition is nice, albeit maybe over-detailed. Good job, nonetheless.
By the way, please fix the background transparency next to his hands.