I think you're giving them way too much credit. At least compared to the credit you're not giving Batman's villians.
Pretending to be a corpse in no way alters breathing. He could simply harden a part of the symbiot around his neck and no pulse would be noticeable.
You are severely underestimated the time that it takes Batman to devise a plan. His thought process has been described as "taking giant leaps in logic." He is the worlds best detective and basically nothing eludes him for long.