[MetroMPG]

(Hope you don't mind I moved your question to a separate thread...)

First question is whether you can safely coast your automatic in N with the engine off without potentially damaging the transmission. Cars designed to be "flat towed" (which maintain lubrication) can do this.

It makes no real difference to the engine itself (mechanically) whether you stop it via the key or a kill circuit.

But you're right in that you'll have to key start every time, so whether it's worth adding a switch (assuming it's OK for your transmission) is debatable.