Mostly agree with you Fenwick, except it the South "waits" until around 1873 or so the development of cheap barbed wire will give defending armies a rather large boost. Repeating rifles and Gatling guns likewise favor the defender (though improvements in artillery somewhat neutralizes these advantages).
If the South simply attempted to defend its own turf and forced the North to be the attacker in major set piece battles, it could expect to inflict massive casualties upon the North, which could undermine public support of the war effort. Overall I think the technological advancements of an additional 15 years would slightly favor the South, but they would probably be over-come by its lack of industrialization and population.
Of course, if there is no Civil War by 1876 or so I think there is a pretty good shot that the US can avoid it all together. Cotton prices started falling quite precipitously in the 1870's, and this trend continued for the rest of the century. This was partly due to the cultivation of Cotton in Egypt and India as a result of the ACW, but at least some of that cultivation would take place anyway. Low cotton prices makes chattel slavery rather less economical, and might make slavery less worth fighting over for at least some Southern Planters outside of S.C.