Diesel remains far more economic that petrol when you do the sums that count and provided you actually need to drive thousands of kilometres rather than thousands of metres. The bowser price per litre comparison will give a misleading picture because what counts is kilometres per dollar. The superior fuel economy of compression ignition motors gives diesel a substantial edge over petrol.
The price hike in diesel was driven in large measure by the move to low sulfur fuel.
LPG has always been a commodity in limited supply and we are past the point when gluts of LPG are common. Like other fossil carbon based fuels, we need price incentives if we are to reduce pollution from LPG combustion. Subsidising LPG would have the opposite effect. No sane government would give the idea serious consideration.
90% of the population accept the scientific reality of global warming.
