Does achieving Maturity Level 2 (ML 2) require that all the processes for the ML 2 Process Areas be developed and documented? Can we achieve ML2 if we have artifacts for the practices but not the documented processes?
The short answer is no. You won't meet Generic Practice (GP) 2.2 without documented processes.
When you read the definition of GP 2.2 Establish and maintain the plan for performing the process, the first item in the bulleted list is process description and the second sub-practice is define and document the process description. If the process isn’t documented, then you cannot perform GP 2.9 Objectively evaluate adherence of the process against its process description, standards, and procedures, and address noncompliance (emphasis added). The CMMI glossary defines Process Description as a documented expression of a set of activities performed to achieve a given purpose that provides an operational definition of the major components of a process (emphasis added). And ML 2 is all about institutionalizing each Process Area as a managed process, which embodies all 10 GPs. So I don’t see how it is possible NOT to document your processes and yet expect to be ML 2. Planning the use of an unwritten tribal process is still ML 1, though better than being completely ad hoc in your approach.