From my understanding, if the 18650 light would accept cr123s- that means you would have a wider voltage range and will loose efficiency. As for putting a dummy cell plus a cr123, that would not be a great idea- if you go to a level that has too much draw- you can damage the cell and or light.
gt390, while an aaa headlamp might be a bit lighter, the size difference is negligible compared to the boosted performance from an aa headlamp.
I agree that the aaa lights are convenient to have on you, I have a couple of aaa lights with eneloops in my power outage kit.