What you are proposing with the EVCS would be a technical violation of 705.12, since it probably has a label on it saying
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Also, if the Powerwalls were set to grid charge, while the car was charging it would overload the 100A feeder conductor most likely.
Finally I do not really understand what is happening here:
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It appears the PW Generation subpanel is connected inside the MSP with some kind of tap? Maybe they used the factory line side tap location that some combo panels have, to backfeed solar into the bus, then used it for the PW Generation subpanel? Seems a bit questionable, and I'd probably go to the MSP with my new EV circuit.