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Some valid observations and questions.
On the transmission/bump starting phenomenon, here is the deal. The Automatic transmission's in use, typically have one pump, which runs off of the front main shaft on the transmission. If the engine was running and you leave the car in gear, so the pump continues to turn, then the engine is still turning over, with no fuel being injected. However, once the car speed slows down to where it tries to downshift, the electronics are off, the oil pressure drops and at some point you have the potential for slippage in the transmission.
If, in most cars, with FI, if your in gear, you let of the pedal completely, the ECU will turn off the injectors, so no fuel is flowing. Once your engine RPM gets down to around 1200, or so, the ECU starts firing the injectors again, so their is a smooth transition and the engine is now available to turn the pump on the input shaft of the transmission.
The amount of fuel savings is basically the same, but you haven't placed your transmission in a scenario where it might get a little low on oil pressure. If it gets low on pressure, you might not notice it, right away, but you can be assured you have some degree of accelerated wear on the clutch's and at some point it will fail, sooner, rather than later.
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