1) Diesel: I wouldn't bother taking a spare can of diesel. Although I've not driven in Norway (so there may be something different there) I've done two particularly long driving holidays in my life. One from Melbourne in Australia to Darwin through the desert outback roads (Birdsville Track, Oodnadatta Track). And another long trip from Chennai (Madras) in the South of India back to the UK via India, Pakistan, Iran, Turkey, Balkans, Italy, France. On both trips I took a spare can and never used it for its intended purpose. Was useful for buying cheap contraband Iranian petrol in border areas (Pakistan / Eastern Turkey) but otherwise was a "waste of space". Was driving a 'normal' car on 'normal' roads. Nothing outlandish.
2) yes I would take a spare gas cylinder - you could get a camping gaz 904 (half the size of a 907) as it means you can use the "in use" cylinder until empty and aren't reliant on swapping something that's not nearly empty.
Difference between 1) and 2) is that you can fill up a partially empty tank of diesel but not so straightforward with camping gaz to do that
3) Loads of small cookers around - both using gas & electricity but then you're back to the 'space' issue.
Bet you'll get lots of differing opinions on this one.
EDIT - the vehicle I was driving (a 1972 Holden HQ Kingswood) had a range of about 350 miles on a tank
