Cost of Reserved Instances when the AZ fails

The following document states that payment is made regardless of actual use, so it may be that AZ failure will also result in payment.
https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/concepts-reserved-instances-application.html

All Reserved Instances provide you with a discount compared to On-Demand pricing. With Reserved Instances, you pay for the entire term regardless of actual use. You can choose to pay for your Reserved Instance upfront, partially upfront, or monthly, depending on the payment option specified for the Reserved Instance.

I think you should open a case with AWS support with "Account and billing" to be sure.

Unfortunately, the discount is not applied in different AZs in case of AZ failure for Zonal Reserved Instances.
Savings Plans do not have capacity reservations, but they can be used with on-demand capacity reservations, which may be more suitable in your case.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/capacity-reservations-pricing-billing.html#capacity-reservations-pricing

Billing discounts for Savings Plans and Regional Reserved Instances apply to Capacity Reservations. For more information, see Billing discounts.