Hi Greybeard,
I think its not possible to answer your question on how kerises might have been used during WWII. I guess it really depends on how the owner wants to use it. I heard in recent times, some guy tried to use a keris to open a lidded tin, and snapped the tip of the keris.
But from the description you have made, I doubt it is a Malay blade at all. Even corrosion is a sign of artificial aging. Nagas - hmm... we've seen lots of new made kerises with nagas carved into them. And the claim that it is an execution keris - well, I guess if it can cut, it can kill.
But seriously, execution by keris is usually done using keris panjang (a form of long keris), and reserved for high status people.