a) You are very unlikely to get in.
b) Even with Harvard's subsidised fees, you will still need to cover your living costs (including any flights and ~$3500 per year medical insurance - welcome to the US!) with no loans at all, is my understanding.
c) Graduate entry medicine is more competitive. This will make it much harder for you to get into medicine. Not "more of a challenge" - I mean raw chance of failure is significantly higher.