Where Longstreet was truly late was at 2nd Manassas. But that's another thread.
I think the blame for the loss of life rests soley on Lee who refused Longstreet's plea to allow the Army to sweep up and behind the Union army. Lee would not allow it... therefore when Hood pleaded with Longstreet to do the same thing, Longstreet would not allow it.
I don't think Jackson would have a problem authorizing a late afternoon attack. He did so at Chancellorsville for example. I think Jackson combined with Longstreet could have persuaded Lee that his present course of action was the wrong tactic for the battlefield.
Lee's plan called for a wave of attacks (fancy French name - don't know it, sorry) against the whole of the Union Army's front. This happened, but not to the degree that Lee had hoped. I think this strategy is the reason why Hood's attack began at 4:00 and McLaws at 5:30.