Yes, you do need to put a payment method on file when prompted by discord to redeem your code. This is normal since you are redeeming a subscription plan, but you can cancel the plan in settings to prevent being charged after the three months.
No, unfortunately these codes do not stack. So, if you buy one, you can only use one. If you buy two, you can share that second code with a friend that doesn't have nitro.
Yes, this is legit. These links are directly from discord. They are not stolen codes, and they were obtained legally.
No, you will not be able to get a refund. Due to the nature of the items being codes, I wouldn't know if you redeemed the code or not, so when purchasing please read everything so you absolutely know what you'll be receiving and what you can and can't do.