My post was meant to be dumbed down because that's what was requested. I described a DC motor because it exhibits the relevant behavior and is much simpler to understand. You'll note that I didn't describe regen because, as you point out, that is handled with phase control in a 3-phase motor (which includes the new model 3 SRPPM, brushless-DC, induction and some other motors). I've designed several 3-phase motor controllers and I can assure that (1) it is very easy to control the phase at full amplitude to maximize driving power and (2) the limiting factor at high speed is back-EMF. The curves above showing the torque drop at lower RPMS for lower SOC are perfectly consistent with backEMF being the cause. Switched reluctance does nothing to reduce back-EMF, but you can play a trick with it to reduce the parasitic inductance which is another (smaller) thing which limits currents at high speed due to insufficient supply voltage. But back-EMF is inescapable. Literally in order for electrical work to be converted to mechanical work, the winding current must act against a back-EMF; if it does not, there is no electrical work (except heating). It is a matter of physics and it is true regardless of any wave shape that you apply.The characteristics that you describe seem to match a DC motor, especially the part about the back EMF limiting motor speed. DC motors are also generators, where while you increase motor a speed, the generated voltage will eventually match the battery voltage and the motor speed will thuI've s level off.
In a variable inverter controlled AC motor like in a Tesla, this is not the case. The only way to make a Tesla motor counter balance the battery and therefore act as a generator is to shift the phases, which is what happens when you take your foot of the accelerator and regenerate.
I may be going out on a limb here now but I think that the leveling off of power as you reach higher rpm is in the Mountainpass diagram above is due not to back emf, but rather to the inverter's inability to supply full tension through a whole revolution while also shifting phases at high speed.
I agree with your post that both rear motors will have the same gear ratio. Since today's Model S is about 10:1, with two motors we will probably see the somewhere near 5:1 with sustained wheel torque for the reason the @mongo explained. That probably lets Tesla delete the intermediate gear so there will only be the pinion and and the ring gear, meaning higher efficiency and lower production cost. Four cogged wheels total for the rear drivetrain. Current Teslas have 4 wheels in the reduction gear and another 4 in the differential for a total of 8, so the Plaid powertrain will be simpler, probably more reliable and cheaper to make.