Hi Devraj,
There are no standard way to achieve the requirement. But, you can use batch characteristics and Batch Derivation with little ABAP code to cater this requirement.
Maintain the certificate validity and other parameters in batch characteristics and with user exits in batch derivation you can fulfill the customer needs.
If you can share more details about the "2) Different customers have different certificate validity requirements for the same products." then it would be much helpful.
Regards,
Dipeshkumar Bhavsar