Yes, I can see that with a diesel inboard you'll have to keep a seperate fuel supply, we have 4 stroke main and aux so no so much of a problem. I'd entirely agree about the reliability of a well maintained diesel. We've never had a problem in over 50k miles on the yacht with the diesel not firing up and working, unlike our four stroke on Hilda Maud.....
Still for info, regarding range, my 8hp does less than 2 litre an hour so you'd get back from mid channel on 25l, albeit v slowly.
Whilst not wishing to start an argument the bit about petrol going off if not used is a bit of an old wives tale. Yes, it will if exposed to air but in a sealed container not so. There are plenty of instances of people keeping and using petrol that is 5 or more years old.
Still, best of luck whatever you decide to do.