No, Capacity Reservations do not guarantee lower pricing like Reserved Instances (RIs).
Yes, an EC2 instance can fail to use an unused Capacity Reservation in the same Availability Zone if certain matching criteria are not met.
Yes, an instance type mismatch can absolutely prevent an EC2 instance from using an available Capacity Reservation.
Yes, a Capacity Reservation will still incur charges even if no instance is using it.
Cold HDD (sc1)