vn S1 manual backfires on gas constantly

[defecat0r]

I would say that you are close. This is how i see it:
The problem arises because of poor ignition with this wasted spark setup. Wasted spark means, as you say, with 3 coils and 6 cylinders, when one cylinder is firing at TDC on the compression stroke, another is firing at near TDC on the exhaust stroke.
LPG is harder on the ignition system than petrol, if the ignition system isn't up to scratch the engine will miss.
So here's what happens:
1/ engine compresses the LPG/air mix to TDC, plug fails to fire (misses),
2/ power stroke(with no power because it missed)
3/ then the exhaust stroke pumps most of the unburnt fuel out. Now the wasted spark comes into it, the plug fires again now with the piston at TDC on the exhaust stroke and ignites the UNBURNT fuel/air an INSTANT before the inlet valve is about to open for the inlet stroke. Up goes everything in the inlet and BOOM your airbox is now transformed into tiny splinters of flying plastic

I gap my bpr6efs plugs down to 1-1.1mm from the default 1.5mm to make it a bit easier on it too

 

[Circlotron]

defecat0r, I think you are on the money.
In a distributor or coil-per-plug ignition the hot centre electrode of the plug is driven negative because being hot it emits electrons better. In a wasted spark setup one of the pair of plugs will always get driven positive and so it's arcing voltage will be higher than normal. Not sure about the voltage needed to fire it though. That may also contribute to the problem.
Thermionic emission - Wikipedia, the free encyclopedia