I don't (and can't!) dispute any of the technical points you've made, but I wanna reiterate something on the Guilty Gear point: they never said "we are chasing the same aesthetic as Xrd", they merely cited it as a broad example of a 2.5D game that looks and feels suitably "authentic", along with some other games like Strider, and when I say "they" I mostly mean Fangamer or Ben Judd or whoever's writing their PR copy, not anyone relevant.
People are riding the Xrd comparison way too hard and I don't think it's to anyone's benefit.
My point is that you cannot do cell shading in UE4 with lighting enabled. You can do cartoonish visuals, of course, but not something that resembles 2D due to the way the lit materials work. Well, you *can*, but that would require either hacking the physically based deferred renderer as to change the entire BDRF or adding a new "shading model", but you'd probably end up with crappy looking post process cell shading. GGXrd's way is much more elegant and actually gives artists control over the cell shading output. It's also damn easy to program.
I just wonder if the game will really need to be cut in any way... if smash could handle 60fps on 1080p, I don't see why this couldn't
Smash runs at spitting distance from dropping the framerate, however. You can get slowdowns in many levels by pausing the game and zooming the camera a little bit, for example, since it makes some effects cover more pixels of the screen than they do during gameplay. Every object, transparency and foliage was placed and moved around to make it possible. It's also not a port of a game from higher end systems.