Evidently the elder Moses Copeland died about 1852 at Mobile. That leaves only the younger Moses Copeland, who served in the Mobile company. I made a mistake about his company letter, which should have been "E" and not "I". Company "E", the company which came from Mobile, was called the "Humphries Dragoons."
Thomas Copeland appears on the 1850 and 1870 census of Mobile. He's a bit younger than we might expect, but has no war record. In 1850 he's living in the household of Isom Copeland, born about 1788.
The earliest census of Alabama was taken in 1820, a year after Alabama became a state. Names of free persons other than head of household were reported for the first time in 1850.
You asked about the connection to Washington County. As mentioned in my earlier post, the elder Moses Copeland remarried in Washington County AL in 1835. In 1830 he lived in Covington County MS; by 1840 he settled in Mobile. Perhaps there was a connection to Washington County through his 2nd wife's Edwards family. At any rate, the Copelands first appear there after 1870 and before 1880.