2.) The primary distinction between prospective and retrospective payments is that prospective payment is when payer dictates what is necessary for each patient and at what cost these procedures will be reimbursed under. In contrast, retrospective payments are just same as or "fee-for-service" compensations that maximize the freedom of patients and providers to decide what procedures are best for each individual. Consequently, the main different between both is that one has a freedom of choice, while other have to follow procedures and protocols to be reimbursed.
3.) I was looking everywhere to answer this question and couldn’t find exact answer. Therefore, I am providing my own answer the way I understand it. To my understanding unreimbursed cost in Medicare is the difference between governmental settlement and the hospital's actual cost. So that means whatever Medicare does not pay to the hospital or provider, the patient is responsible for the difference unless he or she has a supplemental insurance that could kick in as a secondary to Medicare.
Reference
Johnson,W.(n.d) Health Care Reimbursement Issues.