In response to your questions I can offer the following:

I believe it is Dunn’s Hill, headquarters for Robert E. Lee and later A.P. Hill in overseeing defense of Petersburg. As to a particular engagement near there in early August 1864, I’m not sure there was anything of note. Of course, in the trenches around Petersburg all you had to do to be shot was expose a portion of your body, there didn’t have to be a battle. Later in the month there was action at Globe Tavern along the Weldon Railroad.

Census records in 1870 and 1880 (Bartow, and Gwinnett counties, respectively) show that William H. Howell was born in Georgia, not Ireland.

The Jackson Guards were Company B of the 19th Georgia Regiment, which was in Colquitt’s Brigade 1863-1865.

In the 1860 Fulton County, Georgia census there is a 17 year old laborer, William Howell, shown in the household of Stephen Howell, 40, laborer, in the Oak Grove District. Fulton County is the home of the Jackson Guards.

You’re not the only one looking for parents of William H. Howell, see:
http://genforum.genealogy.com/cgi-bin/pageload.cgi?Cordelia::howell::7354.html
http://boards.rootsweb.com/surnames.howell/4895/mb.ashx

The Southern Cross was presented by the United Daughters of the Confederacy after an application. So, if you didn’t apply, you didn’t get one. See http://www.hqudc.org/so_cross/

I wish you well in your search.
George Walker