Since I no longer have a computer and am relying on the kindness of friends and am away from home, I can't give complete information about Custer's report of Gettysburg.
I remember that at either a Gettysburg conference or a Monroe conference the attendees received a handout including the full report. This was possible because a copy of the report must have been sent to the state of Michigan and was included in a state publication. The publication may have been entitled something like the "Annals of Michigan" and it seems to have been a compilation of reports of Michigan units. Separate reports of the regiments of the Michigan Brigade were not included in the handout.
I believe the original report was among the papers lost when Custer's HQ wagon was captured at Trevelyan Station and may never have reached the War Department as part of the brigade's records. I don't think misplaced reports were unusual. I think that when the Official Records were compiled former officers were queried for their reports or sent their copies in.
Whitaker may have had access to a personal copy that Custer saved. I compared the handout copy to the copy in Whitaker's biography, and as I recall it was word for word except near the end when Whitaker cut the report short.
Custer's Gettysburg report was lost during the action near Buckland Mills in Oct 1863. This is very fortunate because I cam never spell "Trevellian" correctly, though I'm an ace with "Buckland" and "Mills." The title of the Publication is "Michigan In the War." The material was compiled by or under the direction of John Robertson, the Adjutant General (presumably of Michigan). While the text of Custer's Gettysburg report dated 22 Aug 1863 is given complete, the usual heading, location, date, and salutation are not included, neither are Custer's signature line nor that of his Assistant Adjutant General. Other excerpts from reports of Michigan Brigade officers are included, but it is not clear if they where made in 1863 or were submitted much later. Russell Alger, Col of the 5th Mich Cav made his report in 1880 to the U. S. War Department on its request. Also included in the Michigan publications were quotes from newspapers. The history of the Michigan Brigade regiments occupied p-573 -613 of the above publication.