Fuel contamination and other basic survival instincts

This is purely theoretical - it is not something I am planning for next
Saturday :

If you are to fly over an expanse of water or other inhospitible
terrain, for a distance long enough to require re-fueling immediately
prior (US-Europe ferry, for example) - You watch to see that they put
the right fuel in your plane, but you can really only see what's
stenciled on the side of the truck, not what's inside (call me paranoid
- what if the line guy is someone I owe money to). How long do you have
to fly before a fuel contamination problem manifests itself? Does this
depend on how empty your tanks were? I'm guessing no - I'm guessing this
type of problem - any serious fuel contamination problem - would show up
within minutes. Anyone really know this. Anyone here learn this the hard
way?

I've also noted, though far less certain, a number of vacuum or AI
failures take place within the first minutes of flight, not to mention
engines throwing rods etc. You can see what I'm getting at - how useful
a survival instinct would it be to fly, say 20 minutes, up the Icelandic
coast, then double back before setting out across the great void?
(Obviously you've already taken the other measures, and you are wearing
your survival gear etc).

Another question : I read an official accident report (not a newspaper
report) in which ground personnel refilled a small plane's oxygen tanks
with compressed air before a mountain crossing. The pilot died in the
ensuing accident, but miraculously the single passenger survived to
confirm the oxygen-related unconciousness of the pilot as the cause of
the crash. What could this pilot have done to defend against this "rare
but not impossible" event?

We assume that in aviation - unlike on the road - we may kill ourselves
by our own mistakes, but the likliehood of being killed by the mistakes
of others is very far removed. This is certainly justified, however very
far removed does not mean impossible, and our preventive measures should
try to preclude suce events if we can.

G Faris