Glad it is working. While I am not by any means an EE (and may be revealing a lack of knowledge here), I do have a few questions that might be worth considering.
First 12VAC is quite notably different from 18VDC (although I do agree that the peak voltage of the former is around 18V), and the latter has quite a bit more power than the former. If 12VDC is run through a diode bridge and capacitor I believe you will end up with something very much like 12VDC. Obviously you don't have instant death, so that indicates that 18VDC isn't terribly far out of tolerance.
Running the LEDs at 18VDC I would try to be aware of extra heat build-up, and reduced battery life from expected life for the battery. Of course you could just measure power consumption at 12VDC and 18VDC to see if there is a notable increase. Without a spec sheet permitting 18VDC operation, I would be inclined towards carrying a spare bulb in case one gets fried, although it looks like you already have two "spares" with you
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