Of course a lot depends on leadership. If Lee is now too old and infirm for the command, any thoughts on who gets it, and how he'd perform?
Brings up an interesting point if who is Commanding General the US Army in this later Civil War.
Winfeild Scott was just shy of his 80th birthday when he died in OTL in 1866, but was so fat and overweight he would have been mustered out anyway.
Besides him the General officers were David E Twiggs and Robert Patterson. Plus some other guy. All of them in their mid-to-late 70s.
Next generation down I think is clearly Lee, Joseph E Johnston, Albert Sidney Johnston. Bearing in mind that Lee is a lieutenant colonel but clearly the favoured man by Scott...he could be the Commanding General in 1869.
Does he still resign and go South...