I am not good at identifying different types of worms.
But I wonder: could she have EATEN those worms, and then puked them up?
I know that dogs like to eat many kinds of things, including poop from other animals. So if she ate poop from a different animal with worms, she might puke up those...
If he likes it, and you have some, I would certainly feed him that until it is gone.
Maybe he just likes variety? If he finishes off the Purina puppy chow, you could see if he likes the previous food again afterward.
Or maybe there is something specific about the puppy chow that he likes, and...
I don't know enough about "balanced" dog training, or any other school of dog training, to be sure of how they differ from each other. But any dog trainer has the potential to see things you haven't noticed, and then you can decide what to do with the information.
You might just call one up on...
Did he get a physical checkup?
As for whether meds would change his personality completely-- I am not a vet, but I am certain that not all meds are the same. A painkiller would most likely have different effects than an anti-anxiety medication, and an antibiotic for an infection or insulin for...
Just to check the obvious: does he have any physical problems? Maybe take him to a vet for a checkup? Anything that hurts can make a person or animal act differently than usual.
Have there been any big changes in his life lately? New schedule? Different people or animals around? Someone new...
Can you weigh him at home?
A typical bathroom scale works, if you can pick him up and weigh yourself both with and without him. (I've used that method for dog, cat, chicken, and baby human. The weights might be a few ounces off, but still very handy if you just want a ballpark number.)
You can put ordinary dry dog food in a dish with water, and freeze that. (Not as much fun as the other things you did, but sometimes handy as a backup plan for when you can't think of anything better.)
Yes, it would have been better if you had not yelled.
But he did what you said (left the duck alone.)
I hope you remembered to tell him he was a good dog (for obeying your command) before you sat down and cried.
This is not an impossible problem, just one you'll have to keep working on for a...