As a side note concerning Douglas Cooper's activities after Pea Ridge, I found a reference to him in "With Fire and Sword, Arkansas 1861-74," Thomas A. DeBlack, University of Arkansas Press (Fayetteville) 2003 which I finished a couple of days ago:

It is in a section concering the continued activities of the Indian Nation troops and the internal conflicts between BG Stan Watie, CSA and Cherokee tribal leader John Ross who "swung both ways" between the Union and the Confederacy. The folowing is quoted from pp 122:

"...Watie, acting in conjunction with white guerrillas, was a constant thorn in the side of Union forces at Fort Smith during 1864. They continually disrupted the critical shipment of supplies from from Little Rock to Fort Smith on the Arkansas River, even capturing a Federal supply boat in June. In addition, they made life hazardous for any Unionist who dared venture far from the confines of the fort.

Watie's efforts were augmented by those of Brig. Gen. Douglas Cooper, whose Indian brigade, consisting mainly of Cherokees and Choctaws, launched a series of raids against isolated Federal units near Fort Smith. In late July Cooper's troops overwhelmed a batalion of the Sixth Kansas at Massard Prairie near Fort Smith, killing 10 and capturing 117. By December the situation for Federal forces at Fort Smith was so desperate that government authorities in Washington decided to abandon the post altogether. Arkansas officials pleaded against the decision, arguing that the move would leave thousands of Unionist Arkansans to the "merciless ravages of the bushwhackers and robbers" and cause "the most loyal portion of Arkansas to become a scene of ruin and desolation."

Apparently, Cooper remained quite active well after Pea Ridge. However, I've not found any further references to him after 1864.