What kind of proof do you want? Do a search on propane fumigation (as it is often referered to) and you will find numerous university studies, studies by the department of defense, and independent research activities all of which concluded that... yes, it works. The proof that does it for me, is that it reduces black smoke, which means that the fuel is burned more completely. Now if you want to argue over the chemistry aspects of it, have at it. I'm not hear to debate how it does it, but rather to restate that somehow it does do it, and putting a gas in your intake is a lot safer than a liquid. Those who run propane will tell you that it will not detenate before the diesel fuel, but that if done right it will burn WITH it. So your timing is maintained. If you add too much propane, then yes you can start to get into a situation where your timing can become too advanced, but the actual cause there of I am not sure of.
Back in the day adding gasoline or alcohol to diesel fuel gave the appearance of running better/cleaner because of less black smoke as well. That was found to be false. So I understand your logic, I follow it as well, but have yet to find the explanation as to HOW it works. So my logic is that it's an additional fuel, why not add additional fuel that it allready has in the fuel tank?