Someone mentioned it above, but Lee's vague command on the first day of the battle, to take the high ground "if at all practicable," was the biggest blunder. Had TJ still been in command, he would've taken the initiative to do so. As it was, Ewell was no Stonewall, he had one brigade guarding his left, which was fresh and could've taken Culp's Hill and
Cemetery Hill. Had Stuart been available to guard the flanks, that fresh brigade would've been free.
Would TJ have taken Culp's Hill and Cemetery Hill on the first day of the battle, hmmmm, that's a no brainer. Of course he would've. The high ground would've been in confederate hands before the entire Union army was on scene.
Meade would've either attacked the high ground or retreated further south to a position he had picked prior to the battle. If he was smart, he would done the latter, otherwise it was Fredricksburg all over again.
TJ was well known for taking the initiative and not following Lee's commands to the 't'. This made him a brilliant commander,....leading from the front, as it were. Too bad he was killed.
Re: Stuart, chances are the rebs would've known the whereabouts of the union army and would've been better prepared to meet them. Even on the 2nd day of battle, a cavalry screen would've been handy on the rebel right flank to speed up Longstreet's attack on Devils Den and Little Round Top. Instead, they attacked in the blind and wasted time doubling back on themselves before the main attack. They may have also seen that the way was open to the Union artillery park and supply train from the rear.
oh well, the fog of war will always git ya....
Most would pick Pickett's Charge as THE blunder of Gettysburg, although indeed it was a blunder to send 12,000 Virginians over open ground against massed artillery and entrenched musketry fire, that was pure madness on Lee's part.