Is "act" in the above interchangeable with "behavior?" If not, why not?
No. abandonment is a noun. If you want it to modify behaviour, you must make it a (participle) adjective: child-abandoning behaviour. Regardless, it is the act of abandoning your child that we are talking about here, so I don't think behaviour
I ran into another similar sentence, and I think it uses the wrong wrong "action" in the following. I suppose "act" works better! Correct me if I am wrong. Thanks.
The policeman's courageous action saved the lives of many people.