1. You should enquire with your lawyer (be it pte or HDB) on the correct procedure/timing to settle the bank loan. Countercheck with your bank as well. HDB procedure are very standard (for non-complicated cases), so a call to HDB call centre may be all it takes to clarify your doubt.
2. Did the buyer accept the OTP on or before the option period (stated in the OTP)? To be binding, it must be accompanied with the payment (cash/cheque) as consideration to you (the seller). A prudent buyer would insist that you acknowledge receipt of the Option exercise form and payment, stating the date/time to do so as time is of the essence in OTP/exercise.
Hope the above clarifies.