At what battle did they ever use Longstreet's corp as an anvil?
The terminology has been used by writers figuratively versus literally. Jackson conducted the Shenandoah Campaign, not Longstreet. He then quickly marched east to join Lee for the Seven Days Battle. Jackson led the flanking march at Second Bull Run and then again at Chancellorsville, not Longstreet. True, Longstreet wasn't there, but two of his divisions were and Lee used them as a brick wall and anvil. Had Longstreet been there, I doubt anything would have changed.
Calling Longstreet the "anvil" is not derogatory. At least I've never read anyone using it that way. It was a comparison based on the employment of both men. You can argue that it was luck, geography, timing, etc, but I get the sense that Lee understood that if you want an independent commander for your flanking movement, Jackson was the man.
Also the last part of you post.... infiltration tactics, is as the name say a tactic.
An army commander like Lee had very little to do with the actual infantry tactics.
And nothing even remotely like infiltration tactics was used doing the war. And it have nothing to do with Maneuver Warfare. that is something you do at the operational level.
The terminology is difficult. "Infiltration tactics" did begin, as far as modern military history is concerned, during World War One as a "tactic" . . . . something used at battalion and company level. But it was very quickly adapted by the Germans to Division, Corps and Army level.
Outside of the Western Front (eastern front, the desert), there were many examples of lightning movement, "dislocation" and "strategic paralysis" . . . all of which are the aims of this kind of warfare. So you could see "infiltration tactics" being used in a battalion-level attack by the German Army . . . and at the same time General Allenby was keeping Ottoman armies off-balance by using the same methods.
Therefore, it's not totally not correct to say that "Maneuver Warfare" is strictly "Operational." It has applications at all levels of warfare. (I say "totally correct" because writers are free to define things however they want.)
Throughout history, there have been two types of combat: Attrition and Maneuver. Leaders usually have a tendency to favor one over the other . . . or as I said:
(In later years aka "Blitzkrieg, Deep Battle, Maneuver Warfare, Air/Land Battle, etc.) Some generals have the knack for it: Rommel, Guderian, Tuckachevsky, Patton, etc.
So my real point is that in late May 1863, Lee had that vision of using maneuver/infiltration/deep battle, etc against the North. I think he understood that a deep thrust into the North would paralyze the Union military, shake the confidence of its leaders and end the war . . . maybe even without fighting the Army of the Potomac.
It can be argued that 2nd Manassas is a time where Lee managed to do "maneuver warfare" very well.
Binding the entire enemy force in place with a part of your force... in a defensive fight and then bring the rest of your force down on the enemy flank is something Both Napoleon I and Von Moltke (the elder) would have recognized and approved off.
Since the both knew very well how to do that... so this was nothing new by the time Lee first started to study military tactics.
(so not something that was invented 50 years later)
We're talking apples & oranges.
True . . . the basic form of sending cavalry or fast-marching infantry against the enemy flanks or rear has been used throughout history. But the evolution of it into what we recognize today was actually an "invention" of the 1920s and 30s. It involved actual development and employment of new technology and philosophies, not available to Napoleon or von Moltke . . . or Hannibal or Frederick The Great.
Von Moltke's planned envelopment of the French Army, for example, was totally different from the operation against France in 1940 where a hole was punched into the French defense and then masses of armor pushed through to the English Channel without infantry support. In fact, the first German plan looked a lot like Von Moltke's plan and was rejected in favor of the "lightning strike."
So there was "invention" and "evolution" between 1920 and 1940 that Lee, Napoleon and von Moltke might have recognized but would have considered it very radical.