I think you have found your man in Napoleonville.
First, Brashear City during the civil war was located at the junction of Bayou Boeuf and the Atchafalaya River which is todays Morgan City and Bayou Boeuf is part of the Intracoastal Waterway.
Second, the players in this drama are all living in Assumption Parish in the 1860 census. Numa Pompaneau is 21 in 4th Ward at the Abermarle Community. Omer Boudreaux is 16 living in Labadieville, Star PO and his father is Paul V 72 and his mother is Scholastie 56. Oscar Hymel is 11 and lives in 6th Ward at Abermarle PO. These kids probably all knew each other.
Omer Boudreaux first joins the 1st Battalion in Pensacola then disbands in Virginia in May 1862. Omer then turns back up in La and joins Co H 2nd La Cav under J B Whittington which was raised in Assumption Parish and he serves in this company for the remainder of the war. Art Bergeron's book describes this companies activities as exactly when and where Omer shows up.
The Theriot (correct spelling) plantation is on Bayou Dularge near the town of Theriot 6.5 miles south of Houma on La 315.
I have found a reference to a Pierre Daigle with a plantation overlooking Lake Verret which is 15 miles north of Morgan City near Hwy 70 on the way to Pierre Part. There are numerous unnamed bayous on the topo maps in this region and I am certain that Bayou Chemise is one of them. Next time I am in the area I will ask some old timers I know if they can tell me exactly where it is. Pierre Part is also in assumption parish but the west side of Lake Verret also catches some of St Martin Parish. I guess the most certain way to check this out would be to research the clerk of court or assesors records for a Dailge property on Lake Verret. Looking at the quad maps I would guess the west side because it has higher elevations.
Hope this helps.