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...
