I called both Capital One Mastercard and Scotiabank American Express again. Mastercard says using coupons and promotions ($20 off weekend rental) doesn't void the CDW coverage, but corporate rates does even if the trip is for personal/pleasure purposes. On the other hand Amex said any coupon will void the coverage.

Both said coupons/discounts that are acquired due to the past rentals which those rentals were fully paid by the card are acceptable.

I ask them to send me something in writing and both refused.