It does not matter if you bypass air through the engine or you bypass it next to the engine whatsoever.
With an e-throttle you do not need additional things because you can just use it to bypass as much air as you want without affecting the pedal.
I would think displacement would matter to air going through the motor, no? Not a prob if you go around it. This is where I think the biggest prob would be, ports in the heads are pretty small.
Lets try and forget about the pedal. I think it is safe to say that most here understand the differences between dbw and dbc.
You do not need to close the throttle for engine braking. It is enough to cut the ignition/fuel which has exactly the same effect. You can argue the engine does not pull vacuum then, but what creates engine braking is mostly internal engine friction.
I hear what you are saying and agree with most of it.
And that you can do too by just bypassing air through the engine. The turbo does not spool because of "low rpm" the turbo does not spool because of not enough volume of exhaust gas.
No problem to create said gas by holding the throttle wide open and throttling the engine using other means.
This volume is why I am guessing they use it. You can throttle the motor by other means, but only up until you are giving all shes got.
Being able to circumvent the engine should let you run full tilt, and supplement this volume to keep the turbo in its happy place.
I do this stuff for a living, I have experience doing it both ways. You can keep this up, but first I recommend a book on basics of internal combustion engines, then a lot of stuff will be much clearer to you without having to spoonfeed it to you bit by bit.
I dont doubt you know your shit here, but there are reasons top level teams with unlimited budgets are currently running this.
The books and spoonfeeding comments are a little uncalled for, especially from someone who thinks friction is what primarily slows a car down when engine braking
(just ribbin ya, I know you know better)