To your point about Corinth and the possibility of those wounded dying at Iuka, indeed I've identified a good number of these men who succumbed to their Corinth/Hatchie wounds and died in hospital at Iuka from Oct-Dec 1862 clearly listed as prisoners captured during those actions. These men are not considered to be candidates for the mass burial at Shady Grove due to the dates of their deaths; I'll explain why.
The Union Provost Marshal at Iuka, Capt. William Wiles and his detail, apparently had finished the mass interment before the end of Sept 1862. A couple of sources (the wonderful book by Cozzens, you cited earlier) and Ben E. Kitchens in his "Rosecrans Meets Price: The Battle of Iuka, Mississippi," both include clues in letters written from Iuka about 10 days after the battle that report the gristly condition of the shallow graves at Iuka rather than any mention of still-unburied bodies (other than draft animals). That means the bulk of the burials were accomplished on or before Sept 30 for the mass grave, and Wiles placed that number at 263. The single marker on the grave itself states the number as 265. (don't know if either of these counts includes Brig. Gen. Lewis Henry Little, who is not buried anywhere near Iuka, but in Baltimore)
(As an aside, the heat in northern Mississippi during Sept-Oct 1862 is well documented by a number of primary sources, which adds yet another motivating factor for burying the quickly decomposing bodies as fast as possible.)
As you observed, Iuka was an important hospital center for the district before the battle ever took place. I believe the 162 "previously prepared" bodies found by Rosecrans' men behind the Methodist Church were hospitalized men sent for care to Iuka earlier and had died there on the eve of battle (I've actually located names for a handful of sick Confederates buried at Iuka as early as March 1862).
Rosecrans' reported 99 Confederate dead on the field in his initial count after the battle. I've relied on Union numbers since they actually held the field after the battle, and the Confederate reports include a number of MIA or "wounded and left" who later turned up as KIA in their early counts.
The 102 now identified men hopefully account for all of Rosecrans' 99 -- plus a few more men who held on a few days before they died and were included in Wiles' final tally along with the Methodist Church's 162.
It's only my opinion, but I don't think anybody wounded Corinth/Hatchie is going to be included in the mass grave. That said, those men ARE buried somewhere at Iuka. In fact, at least one man (Sgt Fairfax Washington of the 38th Mississippi) who was wounded in the fight at Iuka is known to be buried in marked grave in another cemetery there. A marker was placed for him in the McKnight family plot (Oak Grove Cem) by that family who'd cared for him in their home after the battle until his death on 2 Oct 1862.
If you do make the trip, be sure to visit the National Parks Service Civil War Interpretive Center at Corinth, MS. It's an exceptional resource (of course, the mass grave in Shady Grove Cem. on Spring Street in Iuka is also nearby).