I have basically no knowledge on this topic, but my initial reaction before I even read your post was exactly that - there are quite a lot of "chicken and egg" scenarios brought up.
Some of the stuff made sense, some of it was (potentially) fascinating, some of it was quite dubious.
If tapeworms released endorphins, would it also not be a parasite?
parasite (noun): an organism that lives in or on another organism (its host) and benefits by deriving nutrients at the host's expense.