During the early planning of the invasion, however, Stuart had proposed an alternative, striking for the downstream fords of the Potomac and moving east of the mountains. That would put him on the flank and rear of any federals north of the river, disrupt Union communications and supply, and mask confederate intentions by presenting a broader front of invasion. The risk was that he would be separated from the army.
In any event, the key point is that Stuart presented this plan to Lee, who approved it. Stuart was not freelancing. The plan came unglued mainly because the Army of the Potomac, in 1863, was moving a lot faster than it had in 1861/62. The command issues were getting sorted out. Stuart encountered Union infantry marching north from the Rappahannock line, and moving fast. Stuart fell behind schedule because he had to detour south and east to avoid the Federal columns. As it was, he crossed the Potomac at Rowser's Ford just hours after most of the Federals had completed crossing at Edwards Ferry, eight miles upstream opposite Leesburg. With a little better situational awareness, the Federals could have held the downstream fords and prevented Stuart from reaching Gettysburg at all.
The celebrated meeting between Lee and Stuart on July 2 at Gettysburg, in which Lee is supposed to have chastised Stuart, is probably a myth. It apparently originated with a critical piece written by Tom Rosser, who had an anti-Stuart ax to grind, years after the war. This was during the period following Lee's death, when other confederates began to (1) deify R.E. Lee and (2) construct the Lost Cause mythology. Since Lee was deemed infallible, the hagiographers had to blame his subordinates for the disaster in Pennsylvania, with Stuart, Longstreet and Dick Ewell all being targets. To his credit, Longstreet stood his ground and maintained that Lee had erred. This, of course, was correct but inadmissible in the South at that time. Along with becoming a Republican and being a constructive citizen after the war, this is what made Longstreet a turncoat in the eyes of much of the South.
Yet Longstreet was very bitter against Stuart’s disappearance at Gettysburg and criticized him strongly in his memoirs. So it wasn’t just Lee. Granted Lee gave Stuart very vague orders but Stuart was happy to interpret them in a way that would let him take another “glory” ride like he did during the peninsula campaign. Only it didn’t work out so well. I certainly am not defending Lee here, the disaster that was the Gettysburg campaign was all on him but there was plenty of blame to go around.
You may very well be right about the “myth”. I’d not be surprised at all if Stuart was made a scapegoat for Saint Robert’s errors. But in practical terms and as it turned out, anything Stuart might have reported about Union troop movements around Gettysburg would have been worth far more to Lee than the booty he collected in the rich southeast Pennsylvania countryside. One cannot blame him or Lee for enacting some revenge for the depredations of Union forces on Southern resources, but it did blind the Confederate commander to what he actually faced and is an example of how any battle hinges on so many individual decisions.