18V should be fine for this LED, it's rated up to 32V.
Keep it under that and you shouldn't have problems.
One way to test is by hooking up a small light to the system, like a brake light (without the LED installed), turn the light on, and check voltage.
The regulator should be holding at 12V, but as they wear out the voltage gets higher and higher.
Another thing to consider is that the VR is basically creating a path to ground and bleeding off excess electricity (putting a load on the system) until the voltage drops to acceptable parameters.
It's the same as adding a light to the system (a light goes straight to ground too), so if you test the system out without a light burning, you're putting 100% of that load into the VR.
It wasn't designed to run without a light for any length of time, so if you check without a light your voltage will probably read high because you're overtaxing the VR.
The VR only works when you flip the light switch, so if you're not using the lights, it's not doing anything.
Leaving the light switch on when your bulb is burned out can cause the VR to fail.
Odds are, if your system wasn't blowing headlight bulbs left and right, your regulator is working as it should.
Q2W...the whole point to this experiment is to see if ChrisG's LED lights can run on an LT's AC electricity.
Running a rectifier is something we don't want, as there are some lights out now that are supposed to handle a quad's AC lighting system.
Problem is, they're at least twice the cost...so if these LED's will work it'll save everyone a lot of money by not having to rectify the system or buy expensive LED's.