Wes,
I have no answers for your questions. You may post them on the Ms. board. Someone there may know if any records are found for the hospital at Jackson or of the 2nd at Robinette. I don't think officers were normally, moved between companies, but I have seen it in other units, he replaced Capt John W. Hood who resigned due to his health and age. Upon his death he was replaced by 1st Lt. Christopher C. McGinnis (promoted to Capt), who was in Co. G the whole war.
Mike