The two daughters by first marriage are --
Mary Yedell Powell married James Monroe Smith, Oct 8, 1872. He is buried in Garland Cemetery, Butler County AL
Josephene Powell married Evander Melton, Apr 7, 1880. Both are buried in Friendship Cemetery, Pineapple AL.

In 1870 Mary A Powell lived at Smow Hill in Wilcox County AL with three children. These are --
Robert M, age 13;
Martha, age 11;
Frances, age 9;
Dempsey M, age 7.

Robert Powell, 1855 - 1872, buried at Pioneer Cemetery, Greenville AL.
Martha L Powell, born Oct 23, 1859, married Thomas E Salter, Dec 18, 1879. Both are buried at Mount Pleasant Cemetery, Butler County AL.
Frances "Fannie" Powell, born Sept 20, 1861, married Francis Marion Lee, Sept 23, 1880. Both are buried at Holly Grove Cemetery, Claiborne Parrish LA.

Of course you already have an abundance of material on Judge Dempsey Monroe Powell Jr (Mar 16, 1863 - Dec 28, 1949) of Greenville AL.

I don't see a service file for D M Powell in Forrest's original command, but it sounds reasonable based on the Powell residence in Wilcox County.
http://www.archives.alabama.gov/referenc/alamilor/rs4thcav.html

Often a soldier would be sent home on sick furlough and receive a discharge without returning to his command.