If the investigation is past OPM’s due to agency date (ACD), then the agency can request an expedite, I have tried this a number of times and don’t really think it got done any faster. Take into consideration that lots of others are doing the same thing, it all comes down to the agency and prioritization. It sounds like yours is one of those that got caught in transition and OPM has not implemented a process to identify the stragglers out there in order to get them closed. I have several in the same boat and know how frustrating it is.