Now I am back to not being certain of the two PNR vs. two ticket issue and that is a critical difference.
DL and MU would each assign their own PNR on a single ticket. Or, there were two tickets issued and that necessitated two DL PNR's. Or DL could have booked both tickets in a single PNR and MU still would have issued a separate PNR. Without knowing the answer to this for certain, it is impossible to identify the solution, if there is one.
Sounds to me more as though there is one ticket with a DL PNR and a separate MU PNR. In that case, OP would not be eligible for SDC. The glitch with MU about the ticket number appears resolved.
If this is the case, this is far from bad service by DL, simply routine processing of an interline ticket with MU not permitting OLCI.