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I'll admit up front that I'm responding to specific points without having thoroughly read the whole post..
  1. 4 motors (AWD, 1 per wheel).
    • Remove all drive train components
    • Mount motors in axle space center and run CV joints to hubs
    • 4x ~50-70HP motors
    • Will require mods to rear suspension (since it's full axle)
    • Not sure if I can control 4 motors from one inverter / controller
    • Low RPM? This would be direct drive unless there's a gearbox that can go between

Motor RPM thoughts:
I plan to put a couple inches of lift and bigger tires on the project. I know that's counter to EV feng shui but I want this vehicle to be utilitarian as well. Therefore, assuming I go with ~33" tires I'd be looking at about 103.6" circumference, which gives me ~612 turns per mile, or ~713 rpm at 70mph. So direct drive would need to be comfortable at that low RPM or I'd need a gear reducer.
Rather than "mods to rear suspension", think of this as "complete new rear suspension of a different type"... which can be done, and has been done in both EV conversions and engine-driven vehicles.

No, one inverter cannot run multiple AC motors, particularly when they are not all locked to the same shaft. One DC controller could possibly run multiple brushed DC motors, but it's not a desirable setup. The separate inverters (and thus separate controllers) are expensive but they are what allow you to individually control wheel torque for best handling and off-road performance.

The motors would need to be huge to be workable without the torque multiplication of a gearbox for each one, and I really don't think it's worth considering unless you are willing to get an unusual motor such as the YASA 750R (large-diameter axial-flux low speed high torque design); the $2 million Koenigsegg Regera uses one at each rear wheel. Fortunately, only reduction gearing and no differential is required.
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