This really is kind of curious.A few more questions come to mind...
When Mike answered the questions about his parents' places of birth on the census, did he list France, Canada, Ireland, US?Reason I ask is that the time of his birth coincides exactly with the Irish potato famine, and Casey is a decidedly Irish name.Born at sea would fit in that instance.Even the "French Canada" answer for his birth would then be misleading, as that was a very generic term for Quebec, though there were any number of British, Scottish and Irish (among others) immigrants settling there throughout the 1800s (it was part of the British plan to keep the US from invading and overthrowing the Canadian provincial government by getting as many people as they could...from wherever they could get them...to inhabit the border as tightly as they could.That's why to this day the vast majority of Canadian residents are tightly knit within just a few hours of the US border.)
I can check Loiselle, but it won't be much good if he wasn't French.But what does intrigue me at this point is the idea of excommunication.There are extensive records of this action kept at the diocesan/archdiocesan level, as well as the Vatican--as the Pope (especially at that time) was usually involved in each of these actions.But excommunication is very misunderstood.It isn't the public rebuke that so many people have been led to believe.It's meant to be a time of reflection and renewal where the communicant is basically reeducated so that they can return to the Church in good standing.He wouldn't have had to change his name, he really would just have had to go through the catecism and renounce whatever it was that he did wrong, seeking forgiveness.and they should have explained all of that to him at the time.
Now as for the Casey/Tacey/Tessier question...I guess it goes back to whether he was French or Irish.There was a contingent of Irish who came to North America that was named Tacey and Tache.Obviously separate from the French Tessier, they would none-the-less pick up French traits as they tried to assimilate into their new culture.Whether the name was originally Tessier or not would depend on ethnic group.
Now the last question is what the Church listed on his Sepulcre/Burial records.Does it give any clues as to which parish he was baptized at, who his parents were, or anything like that?
The other thing to check into is what was happening in Maryland at the time that he got sick.I did some cursory checking, and it seemed to be a lull in the war.A month after Gettysburg and a month before the battles of Chickamauga and Chattanooga, there wasn't too much happening.The majority of troops were with General Meade in Virginia. So I guess I'd check into whether Clear Springs might not have been a military hospital, rather than a field hospital.If so, there may be medical records available, which are usually extremely detailed, especially if it was going to be a permanent condition.NARA and the Pentagon can usually point you to the right place, but a good military historian from the area (and there are many) can tell you what was happening there and why...and may even know where some of these records might be archived.
This really is an intriguing guy.I can tell how frustrated you've been, but maybe he left enough clues out there to figure it out...keep the faith (bad pun, I know!)