I think that renting the flat is the best option for you for the following reasons:
1. When someone stays in the apartment, it stays in good working condition; an empty apartment may develop some problems that may go undetected and may actually result in the deterioration of the apartment.
2. Since your apartment is financed to the extent of 85%, the rent you get from the apartment can go towards your monthly EMI thus reducing your burden.
3. It opens up a whole new market of buyers who are interested in buying apartments for renting ; and since yours will already have a tenant, it becomes more attractive for the buyer.
Having said that, it is important to remember that the tenant should be high quality like a corporate , MNC etc.
Your second question was with regard to the settlement of your home loan in case of a sale of the apartment. The process is fairly simple, wherein, the buyer issues two cheques, one favouring the bank to settle the loan outstanding and the other to you for the remaining amount. Of course you would need to provide the prospective buyer with all the necessary documents in advance, such as a letter from the bank listing all the documents held by them.
As regards prepayment, a loan can be prepaid either in part or in full at any given point of time. You can also prepay a loan even when it is only partly disbursed. However, most banks have an upper limit on the number of times a person can prepay his loan in a year as well as on the minimum amount you can prepay each time. Until recently, banks charged a penalty for part or full prepayment. Increased competition has forced most banks to allow repayment without any charges if it is funded from own sources. In case the borrower, is transferring the loan to another lender he will need to pay the full charges.Typically the loan will attract a prepayment penalty if it is prepaid in full by sale of the property that it had financed. The prepayment penalty may be 1-2% of the loan outstanding.