I doubt you will have read much about brevet promotions in the Regular Army in the Official Records of the Civil War. Most of those promotions were probably processed in 1866 and 1867.
Custer was brevetted Maj, Lt Col, and Col in the Regular Army effective the dates of Gettysurg, Yellow Tavern, and Winchester and Brig Gen, Reg Army 13 Mar 1865 (an administrative date for lack of a better term)for Five Forks. He received news of those brevets in the late summer of 1865 while in Texas serving as a Maj Gen of Volunteers to which rank he had been promoted eff Apr 1865. That date coincides with the resignation of Ambrose Burnside (only 70 full rank Maj Gens of Volunteers were authorized). His brevet as Maj Gen of Vols was effective with the date of Cedar Creek and appears to have carried with it the right to command in his brevet rank. His brevet as Maj Gen in the Regular Army was effective 13 Mar 1865 (again an administrative date) for the Appomattox Campaign. The brevet of Maj Gen was actually processed about the same time as his appointment as Lt Col of the 7th Cav, and his is listed as a Bvt Maj Gen USA in the 1867 Army Register.
Custer's appointment as a full rank Maj Gen of Volunteers expired 1 Feb 1866, and accordingly he was mustered of the Volunteers. The same expiration date applied to Maj Gens Stanley, Merritt, and Mower which was 2 weeks after that of Maj Gens Hunter, Rosecrans, Parke, Granger, Sykes, Pleasonton, A. Smith, Gibbon, Osterhaus, Crook, Hazen, Griffin, Emory, and Potter (Heitman, see below).
I think it was understood that the Volunteers were what might be called today a seperate component of the U. S. Army that would not be needed much beyond the end of hostilities, but I have never read the term "temporary" applied to officers or men of any of the volunteer regiments mustered into federal service.
You seem to use "temporary" as if you were talking about an acting sergeant in a basic training company. You seem to be saying Custer was only a temporary officer as if in amounted to nothing or was some kind of joke.
No brevet was awarded to Custer after his death. After 1866, there were no brevets left to give him. This was the case with many other senior officers of the Regular Army.
You may find Custer's basic record, including the dates of his brevets, in Francis B. Heitman's "Historical Register and Dictionary of the United States Army," p. 348. If you have connections at West Point, they may have Cullam and the yearly Army Registers which should contain the information as well, ditto Carlisle. The dates are the effective dates and not the dates the actual certificates were signed or reached Custer. Further details on Custer's various promotions may be found in Roger L. Williams' "Military Register of Custer's Last Command." Good information on brevet rank can be found in the excellent introduction of Roger D. Hunt and Jack R. Brown's "Brevet Brigadier Generals in Blue."
To compare Custer's brevets to Harland Sanders colonecy is, I think, meant to be nasty. Whatever is on Col Sanders' headstone is a matter among Sanders, the State of Kentucky (I assume he was made a Kentucky Colonel) and the Lord. You would do better to compare Custer's brevets to the DSC, DSM, Silver Star, Bronze Star, Air Medal, and Purple Heart. These decorations, had they existed in his day, might have come Custer's way.
Information regarding desertions in the 7th Cav can be found at the Message Board of the Little Big Horn Associates via www.thelbha.org. Under "Independent Research" you'll find 2 threads dealing with desertion.