I have extreme chewers. It is Kong toys and rope only. I worked for a rope distribution company and bought 600' of 100% cotton rope. I make their toys. They chew, play tug of war with each other and it is okay shrewd. It is all natural. The Kong toys are almost indestructible but they are always supervised.
I don't give them squeak toys because the sound of the squeak is also the sound an animal makes as it is dying. I don't need to encourage "killer" behavior. I've got one that has chased and I believe killed vermin.
Mine play the game, whatever toy you've got (as in the other dog) I want to play with it. I don't care how many other cool toys are right there.
So, as to why they have decided to protect the stuffed dog, there could be many different reasons. If any of yours are females, it could be a false pregnancy issue. They may have adopted it (which seems your pack has done.) So, you could name it and they will learn it's name. One day, they may turn on it as the omega of the and kill it (shread it) but until that happens, you've got five instead of four dogs, lol. It is just one less to feed and clean up after.