Have you researched what weapons Cushing may have actually carried? Is there any documentation available that ties the pistol to the user - be it serial number assignment, family documentation, etc?
IMO, one shouldn't rely the engraving type set alone for a determination of authenticity unless you can match the type set exactly to another similarly or identically engraved handgun. Nor would I rely solely on patina or wear as an indicator of authenticity - what if the pistol had always been kept in the family, cared for and recently cleaned? What if it had been kept in a museum or perhaps in a curators collection? Perhaps it never saw service and wasn't engraved until after the war, perhaps as a GAR commemorative, thus having age to it but not any war service wear typically found on a pistol from the ACW?
Knowing Cushing was a West Pointer, perhaps they may have some additional info regarding the pistol in their historical archives. For instance, 2 of Patton's pistols are at the VMI museum, with family documentation. They are on loan by the family and may one day have to be returned. Slight as it may be, maybe the pistol was once displayed at the Point.
I think there are just too many "what if's" at the moment to determine if the inscription itself is wartime, post-war engraved or simply fake. Let us know if you have anything on the lineage and documentation showing its ownership or use...we are all now as curious as you!