You dont understand how fine wire electrode plugs work. They, with the proper wide gap, transfer more electrical energy, making for a more reliable "light off" of the fuel-air mixture. Iridium alloy withstands the heat much better, allowing for the fine wire, than traditional nickel, that need to be relatively big and thick. Platinum is in between the two in cost and longevity.

On the NGK site, they explain why LPG engines need a colder heat range. Since it is already a gas in the mixer, there is no cooling effect as there is with evaporating liguid fuels such as gasolne or alcohol.

Jim