A configuration audit is an audit conducted to verify that a configuration item, or a collection of configuration items that make up a baseline, conforms to a specified standard or requirement. FCA and PCA are examples of these.
You can choose whatever works for you. Your questions are the sorts of discussions that usually occur between an organization and their process improvement guides, both internal and external. From, here, we cannot presume to determine precisely how your organization needs to execute the CMMI practices to meet your business objectives. The SEI is not there with you seeing the business environment that may make the question positive or negative in response.
Usually, the FCA and PCS are conducted periodically at an event-base or each phase end.
