If that merchant is not one of the exceptions listed, then yes you will get 5% cashback.
Edit: if the instant discount is given by the SBI bank offer and not by the website, then you might want to see the T&C where it may show that SBI CB card is not eligible to get discount. In this case, you will only get 5% regular cashback
No you can't get 5% cashback if you avail instant discount. Even if they credited the cashback after discount, they will reverse it back after sometime