Lots of trimming

Also, I've never seen oil being more subject to explosion/flash than
gasoline. How does adding oil to fuel make it more volatile?

L8R

Skip

There are 2 parameters to consider these are the flash point - the min
temp at which it will ignite with a spark or flame etc.
This is typically -45F for petrol (gasoline) and +143F for diesel.
Hence as a boat fuel, diesel is a lot 'safer' than gasoline.

However there is also the autoignition temperature - this is the min
temp where the fuel will simply ignite without a spark or flame.
For Petrol this is 495F and for diesel 410F a fair bit cooler.

So too much oil could lower the auto ignition temp to a point where
pre-ignition happens - not good for your engine.

However I have no idea how much you need to add to cause this kind of
problem.