I think you can somewhat isolate the upper from lower part of chest cause otherwise everybody would have a complete chest from just flat pressing.
As far as flat flies i actually like doing a fly style press, hold the dumbbells by the inner part and they will try and "fight you" outwards causing you to put tension on your chest, the purpose is to never straighten your arms so it's like a fly but press at the same time. I tried them the past couple workouts and i get an insane pump and feel like i get more isolation.
I would say flies are made to cause stretch position tension towards the end of a workout to stimulate the last amount of fibers by the stretch reflex since you usually can't go heavy because of muscular fatigue. They need to be done slowly and avoid tapping the dumbbells together at the top which is kind of stupid considering you lose all tension in your chest. Bring them up just outside of shoulder width, the pump and stimulation are 100x better. Train smart and hard!!