This answer is only based on the information provided in the question and the question background. You stated you came across a modification changing a Cost CLIN to a Firm Fixed Price CLIN so the contract could be closed out quicker.  You also stated it was billed as a Cost CLIN, which means it was a Cost CLIN when the work was done. Then after the contract was completed the Cost CLIN became a Firm Fixed Price CLIN through a modification.  A modification can’t be used to go back in time to call a cost CLIN a Fixed Price CLIN to speed up the contract closeout process.

Your second question is; Does DCMA/DCAA have to review it before you close out the contract because it was billed as a cost reimbursable type CLIN?  Please see Quick closeout procedures at FAR 42.708.

Your third question is; Doesn’t DCMA/DCAA have to review the costs to ensure a fair and reasonable price?  The Contracting Officer is responsible for getting a fair and reasonable price.  They should not award a contract if the price is not fair and reasonable.  If you are asking about an audit please see FAR 4.804-5(a)(12) which states Contract audit is complete.