1. Yes it will. For example if you configure the payment registration module to add a user to a role with expiration days set to 365. When the user make payment and they are not a member of the role it will add them to the role for 365 days. If the user makes payment and they are already member of the role then the module will add 365 days to the expiration date of their role.

2. Yes, users can renew before their role expires and the module will reset their renewal date.