There's a really nice article in the Southern Bivouac, December, 1886
https://babel.hathitrust.org/cgi/pt?id=wu.89062341201&view=1up&seq=480&size=125 and also a condensed version of the same article in B & L, Vol III, p. 663-65 titled The Crisis At Chickamauga, by Gates P. Thurston, Brevet Brigadier-General, U. S. V.
In this article by General Thurston (who was at Chickamauga) I get the impression that, according to Thurston, it was not a forgone conclusion; that if not for the unnecessary re-enforcement of Thomas left flank; Longstreet's attack would have met with a "well organized resistance". If the attack had occurred thirty minutes later (after the actual move of Palmer's Division) a more stout defense to the right of Longstreet's attack could (should?) have prevented the rout of the Union left. I surmise this from what he writes and, more specifically, from the following:
"Briefly, this is the story of the disaster on our right at Chickamauga: We were overwhelmed by numbers; we were beaten in detail. Thirty minutes earlier Longstreet would have met well-organized resistance. Thirty minutes later our marching divisions could have formed beyond his column of attack," B & L, p. 664
In the
Bivouac Thurston mentions Longstreet's confidence in the success of the attack, saying that "it was a physical impossibility for any troops, unprotected by breastworks, to withstand such a column." I don't believe the Union troops could have erected breastworks in one night. IMO, such a compact and powerful force thrown into a specific portion of the Union left would have broken through. What's unknown is how long it would have taken for the attacking column to break through, and how would the defense have reacted to such a breakthrough, if Palmer's division had remained in place.