On a PA28 the fuel primer can leak fuel into the cockpit.
Yes, very true. Also the oil pressure gauge is usually a bourdon tube job which can leak too.
But these things will need an ignition source. I am pretty sure that if you had a
fuel leak into the cockpit, big enough to create a vapour pressure in the right range for ignition, the cockpit be full of fuel and the resulting fire would be massive and totally explosive.
An oil leak is something else but it would be hard to ignite the stuff unless you dropped a fag straight into a puddle of oil.
My money in this case is on a battery going off, or ignition (of paper or the seat material) by a fag.
Or a straight electrical fire, but normally they can be stopped by pulling the main power CB.
where would one stand legally if one jettisoned an object, say smoking laptop for example, which absolutely put the aircraft and POBs in imminent danger, but on its return to earth struck some poor individual on the head possibly killing them? Far fetched I know but such is my luck sometimes!
Extremely unlikely you would hit somebody. I wouldn't worry about it anyway. It's got to go.