Yes, if you take an engine and switch its fuel to one with lower energy content, it will have less power.
But if you use an engine designed and rated for Natural Gas, you will get the right power output. That engine will have more cubic inches than the same horsepower diesel engine.
Yep. The same is true for ethanol.
NG trucks are much more appealing than NG cars:
http://www.freightlinertrucks.com/Trucks/Alternative-Power-Trucks/Natural-Gas
I think the the HPDI (high pressure) technology that is proprietary to Westport makes up for the lesser energy density of NG vs diesel.
Remember your freshman Chemistry:
PV=nRT (pressure * volume = number of moles * constant* temperature)
(Number of moles is the quantity of the gas.)
So with high pressure you can pack more fuel into the cylinder for combustion.