Yes, to make a stock inverter from a car work you need to fake a lot of traffic that would have come from other parts of the car - throttle, ECU, battery, etc. If it doesn't see the traffic it expects you get nothing.I'm certainly no expert in the Leaf, but I think you've touched on the problem: for the inverter to think it's still in the Leaf, there is a lot of communication with other components (or something pretending to be those components) is needed.
My understanding is that this is why the people working on the DIY Tesla controller have taken their approach: if you can't convince the original controller that it's still in the original car, and you can't reprogram it, you can build a controller that works with the original inverter but does what you want. Unfortunately, you have to build a controller...
And, yeah, that is why people are even doing DIY Tesla controllers. We have gotten Tesla drive trains to work with the stock hardware but it was difficult and Tesla routinely changes the way traffic is constructed. That requires that you can then flash a known firmware version onto the inverter. That all is tough and complicated so other people make DIY boards to alleviate these issues. But, you get issues that way too - you probably will go through multiple board revisions, you need to code up inverter software yourself or modify something, you might never get things as tuned as Tesla or Nissan did, etc.
For what it's worth, I will likely be trying to make a Leaf inverter and motor work outside of a car soon. It seems there is some interest in doing this and so far no one has managed to do it without a lot of cheating (read: using tons of other OEM parts from the car until it works). I want to do it just inverter/motor on a bench.