Does there need to be a MS-C ADM?
If not, then is the MDA's approval of the LCSP the last ADM needed? (Just enter the sustainment phase?)
If there isn't a MS-C ADM, then how are the tailored set of statutory & regulatory documentation verified as complete?
The short answer is “it depends”.
After reviewing the information provided and getting further information about your situation via phone on 4 March 2019, it seems that your program has several things to consider when determining the best path forward.
A thorough reading of DOD 5000.02 Enclosure 1 as related to a Milestone C decision will help identify the documentation that may be appropriate. In addition, IEEE 15288.2 contains reference information for DoD Technical Reviews and Audits (like Production Readiness Review and Physical Configuration Audit). Although many of the activities referenced will have already occurred, it is a good sanity check to look at what a traditional acquisition would have done and assess what information exists for your effort and what gaps (if any) exist.
As you note, a Lifecycle Sustainment Plan is appropriate. Addressing roles/responsibilities, maintenance concept, spares, facilities, funding, etc. is vital for the continued success of the project. In addition cybersecurity, physical security, and other aspects of program protection should be addressed and documented. If the program has or references a Validated On-Line Threat (VOLT) or System Threat Assessment (STA) it is important to address the impact of changing threat characteristics. How this is done and who is responsible should be addressed.
An appropriate lens through which to consider these ideas is risk management. What promises is the program making and to whom? How will success of the effort be determined? What might occur that would prevent the program from fulfilling its promises (in terms of cost, schedule, performance) going forward. What should be done (if anything) to mitigate these risks?
DAU seeks to provide value to acquisition efforts by serving as a thinking partner and resource to enable program success. Please let us know if we can be of further assistance.