With the additional comma the “gain” doesn’t relate to the “treasure maps” anymore.
Same with the 2nd trait. By adding the last comma it is implied that the troop is immune to all the mentioned stuff but also will devour an enemy troop. Which is not the case.
That’s not how that works. 
“I ate a salad, a steak, and a potato” - it’s just a serial comma. Not needed but everything still has been eaten. Whether it’s there or not doesn’t change the meaning at all.
You’re worrying about something that is really super inconsequential.
Also, English is used in many different countries by many different people. So some countries got rid of it and others didn’t? It’s optional? Well, then just ignore it as it’s not wrong that it’s there.