Cashback of Rs.10K upto 10K aren't taxable. Also cashback o only those entities considers where you have PAN card attached. So by law you need to include it in income as 750x12= 9000 and also if you eraned from other banks. You can also avoid and then they will themselves inform you about it later if they get to know about it. Till then enjoy.
You can overpay CC bill with some banks but not all. ICICI, Axis, PNB are few which allows. I always pay excess amt in Axis as they refund to my savings acc, before stmt generation but not always. Only when have DC spend offer from HDFC