Dennis,
You have done some interesting research. I agree that there were a considerable number of deserters or shirkers in both armies. Accurate data can no doubt be assembled on some units and inference as to numbers could then be made for the armies as a whole at Gettysburg. As to deserters bonding together, I recall a soldier passing a field on his way to the battlefield and commenting on the heads bobbing above the grain like so many turtles in a pond. Some deserters are known to have obtained civilian clothes from sympathetic civilians. I can imagine a deserter randomly picking a numbered enemy unit, or else selecting a unit known to him through the papers or from personal interaction - on the picket lines for instance. So far I am with you, but I have some difficulty with the assimilation of Union deserters among Confederate prisoners. Surely it would not take long for a Confederate soldier to determine the veracity of a concocted cover story. Men from a particular city or county were scattered among the armies in different units, so the average soldier seems to have known quite a bit about other commands and commanders. Organization of the support services (wagon trains, ordnance, medical practices) were so unique that a liar should be easily detected. Whether they would expose a suspected Federal deserter to the prison guards is another matter, but such rumors would spread rapidly by word of mouth among the prisoners, and at the very least one would expect post-war correspondence to detail such instances. I can scarcely imagine Confederate prisoners accepting a Union deserter in their midst owing to an absence of trust; would they not be shunned as a potential spy inserted by the prison authorities? If so, they would live a solitary life during their confinement. But you have shown they did exist. I'm not surprised, because each individual from that vast war has a different story to tell, and if you will forgive the pun, there are at least 50 shades of gray.