I see now in his records that he clearly died on 20 Oct 1862 "in hospital" (right inside the window of time we're investigating for candidates for that mass grave at Iuka).
Do you have any documentation or inkling on the location of the hospital in question? Unfortunately, I can not discern from the record if he was possibly a Prisoner at the time of his death (no POW or Exchange mention is included in his Service Record). Many records I'm encountering in this particular search show a date of death during the Sept-Oct 1862 timeframe often with a variation of the phrase: "in Union hands," for a sick/wounded man who died in the hospital at Iuka as a Prisoner of War from the battles of Iuka (19-20 Sept 1862) and/or Corinth (3-5 Oct 1862).
Others that I'm finding during the same timeframe, (who were not POWs) but who died while in Confederate hospitals at other locations have been eliminated from consideration for that mass grave at Iuka.
The tough part insofar as Needham Temples is concerned is that I can't tell from the available record one way or another just where he died (or his possible POW status) conclusively. The fact that there is such good documentation of his widow's claim leads me to think that he must have been in a Confederate hospital somewhere other than Iuka at the time of his death, but, as I said earlier, it's just too hard to say for certain based on the available paper trial.
Thanks again. We'll keep digging...
-Chip