The whole reason a chain of command exists, both then and now, is because we recognize that it is Humanly impossible for a single man to control everything in a large force. This is particularly true on a large battlefield with multiple engagements occurring, with the communications technology of the time. Lee had issued his orders and it was Longstreet's job as his subordinate to carry them out to the best of his ability. Lee had provided the resources and it was up to Longstreet on Day 3 to properly utilize them.
The Union batteries were already low on ammunition confronting the historical force; more troops means there is more targets than they have capacity to engage so it comes down to an infantry fight. By Day 3, Meade had no deep reserves, everything was already on the line so his ability to counter-attack means leaving somewhere else weak:
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As far as Lee and Longstreet, I don't think it's out of bounds that Longstreet was Lee's favorite and he'd cover for him. We do know he played favorites, after all, and I do think that is a valid critique in of itself. I'm not saying Longstreet was an incompetent either, but I do think his performance on Day 3 needs to be viewed in the same way we consider the Knoxville Campaign for example.