The SOC has nothing to do with integrating the two parts of an employee group together or contracts. It just means that the FAA is satisfied that everyone is doing everything the same way.
Until representation is decided, the PMNW employees working under a CBA will continue under that CBA unless there is a negotiated agreement saying otherwise. DL can't just say "Everyone's DL now so you contract with NW is meaningless." Well, let me rephrase that...DL could try but wouldn't be successful.
As I said earlier, DL could integrate the employee group while still abiding by the PMNW contracts for the PMNW employees. It would add complexity due to the factors you touched on, but it could be done. Whether DL will do that is a question I certainly can't answer. As for your specific question - "If and when DL receives the SOC are they then free to implement DL policy or are the required to abide by the IAM contact work rules? Or both?" - it depends on what you mean by DL policy. After the SOC is issued, policies involving FAR compliance will be the same for both groups - that's required to get the SOC. Contract provisions - work rules, vacation, pay, etc - are completely different and have to be complied with. So I guess the answer is "Both".
Jim