I am afraid that according to enotes regulations I have had to edit your question down to one question alone - multiple questions are not allowed.
You might get different answers to this, but to me, I think the beating of the heart symbolises the guilt of the narrator at some deep and profound level. It is clear that this sound is one that only he can hear - and let us remember how the narrator starts the story by telling us about how his "disease" had "sharpened" his "senses" rather than dulled them. This beating is a sound that he is unable to ignore, has a massive impact on the narrator and becomes ever louder:
I foamed - I raved - I swore! I swung the chair upon which I had been sitting, and grated it upon the boards, but the noise arose over all and continually increased. It grew louder - louder - louder! And still the men chatted pleasantly, and smiled. Was it possible they heard not?
Perhaps we can see the heart-beat then as the sound of the man's conscience, persistent until it is acknowledged and confessed.