An undersized ground in addition to the code-compliant pipe as a ground?
The NEC doesn't directly address this; while you can probably make the case to allow it, chances are that any AHJ would not allow it. Here's why:
When you're doing parallel runs, the ground wire is required to be able to handle ALL of the fault current - except that the NEC modifies this by stating that the ground need not ever be larger than the power conductors in the pipe. If we apply that standard to your situation, I can see a requirement for the ground to be able to handle ALL of the fault current. Maybe, just maybe, the AHJ would allow you to size the ground by the 90 degree column - but that would be his allowing you to vary from the code mandate that smaller circuits use full-size grounds.
Tesla has a point: the NECA "trade practice" standard for installing EMT advises that you avoid the use of 1/2" in industrial settings.
Motors, of themselves, are linear loads. There is only a 'non-linear' issue when there is a frequency drive AND three-phase power. Other non-linear loads can be electronic ballasts, power supplies, and induction equipment. In any event, you deal with these issues by having a larger neutral- the ground wire isn't involved.
